Short Version - It's fundamentally broken because it's too easy for it to end up useless or an outright liability to the Mafia.
Long Verison - Let's start by breaking its effects down into three parts.
1 - Prevent Spies from seeing the targeted Mafia member's target.
2 - Temporarily replace the Mafia target's investigative results with those of the other target.
3 - Visit a member of the Mafia.
Number 1 is the most immediately useful, as it prevents Spies from checking off visits by Framers, Hypnotists, Blackmailers, Consiglieres, and Consorts as not Mafia as well as having the secondary function of allowing the Forger's actions to be covered up, whether they're going with the Godfather/Mafioso or forging a quitter. It's not without its problems, however, as the moment the other Random Mafia goes down, it becomes almost completely worthless as anyone the Godfather or Mafioso targets is usually going to end up dead anyway short of them being jailed, doctored, or protected by a Bodyguard, and in the current ranked role-list you'd want to let the Spy see the Mafia visit the Witch or Executioner so that the town trusts them more, and they thereby become easier to work with/use. But of course, this effect relies on there being a Spy in the first place, so if the town doesn't have that one exact role or if the Mafia has already picked off the Spy or Spies, then none of that matters because this effect is pretty much doing nothing and there's no feedback to let you know that it's doing nothing.
Number 2 is the same problem that the original Framer had. It has no feedback whatsoever, so you don't even know if the investigation adjustment mattered that night, only that can be even worse than the original Framer had it because if you end up disguising your Consigliere as a Vigilante and they get investigated and claim Investigator, the "incorrect" role bracket is going to give them problems. You could ameliorate this by making it so that if the disguised Mafia is visited by a Sheriff or Investigator, both they and the Disguiser get a message saying as such, but the effect still relies on the town having specific roles in order to be relevant, and if there are no Sheriffs or Investigators or the Mafia has already picked them off, then this effect doesn't matter (Framer also needs help about this exact problem even today, but this isn't about that).
Number 3 is where the role completely falls apart. At first blush it seems useful because it allows you to make Spies think that Mafia members aren't Mafia members, but on top of the fact that it, again, requires a Spy to be present in the game or else it's worthless and you don't know that it's worthless because there's no feedback. That's not the main issue, though. The main issue is that this ability will backfire the moment you go down, as if there is a Spy and they're still alive or their Will is up to date, then the Town will know right away to start paying special attention to the living Mafia visits, in particular any that were visited multiple times since the Disguiser has such a limited pool of Mafia to visit, as well as the fact that if the Spy is particularly assertive, they might notice someone keeps getting visited by the Mafia but is talking during the day (so obviously not Blackmailed) but also isn't mentioning being roleblocked or anything else, making it unlikely that there's a Consort or Hypnotist, and then call that out. Then, if they get lynched or executed the town knows that there's a Disguiser, or inversely if you go down first then they know that those repeated Mafia visits were you, and they can nail your allies.
With that all said, I don't believe there's any fixing this role. Reverting it would do no good since the old Disguiser was broken for different reasons (could easily be foiled by signing one's Will and even if it were reverted with the change that it just destroys the target's Will or uses the Disguiser's current Will instead, it probably resulted in a lot of erroneous reports and that would still be an issue), and I can't think of anything that it could reasonably do. The only idea that immediately comes to my mind wouldn't work because it'd play off the original Retributionist (You can read that at the end of this post if you want). That all said, I think it'd be best if the Disguiser were just thrown into the trash. It's already been overhauled once, and trying to stay married to the name of Disguiser for a second overhaul would just make the matter more complicated at best, and at worst you end up with a Disguiser in name only.
That overhaul that would play off the original Retributionist, if you want to know, is to have it so that is once per round the Disguiser is allowed to forge itself as a town role (Neutral and Mafia are not allowed) and "die" at will (also choosing the supposed manner of death by hand), then being able to "resurrect" any following night, being forced to "resurrect" if the rest of the Mafia is wiped out to make sure it goes back into a state where it can be killed for real. While I think this idea sounds really, really cool at its face, I also understand that it would be an extremely passive role even if one tried to make it more active by allowing "dead" Disguisers to hear and speak to the dead chat on top of still being able to see the Mafia chat (though probably not being able to talk to the Mafia until they get up), on top of the fact that it simply can never happen anyway because the old Retributionist will never come back (and, salt in the wound, any round with a real Retributionist would be messy for obvious reasons, regardless of whether or not they were capable of wasting their ability by forcing the Disguiser to get back up). I feel it'd definitely have more utility than the current Disguiser (especially paired up with a Janitor, pulling off a stunt like killing and cleaning the Jailor, "dying" as the Jailor, and then resurrecting would be beautiful, if short lived in its practical applications, I guess doing the same with a Veteran would be more practical), but well, it'll never happen.