Bungie announced that a fix for the bugged state of Falling Guillotine will be arriving on the next reset. The sword should still be top tier for PvE DPS though.
Falling Guillotine has two bugs attached to it but one of them is replicated more often and presents a bigger disruption to Destiny 2. Namely, it can keep dealing damage even after the heavy attack animation has concluded which contributes to a massive damage spike and ability to wipe the floor with even the sturdiest of enemies. Until they slam you away, that is.
Anyway, this issue will be fixed on July 7, 2020, when the weekly reset hits. Despite the bug providing damage, it wasn't appearing consistently and chances are that you will not notice a major difference in your damage output if you haven't been actively trying to glitch the weapon.
The heavy attack will still be great for burst damage dealing, with the tradeoff being the inability to activate it against airborne enemies.
Another glitch that is tied to Falling Guillotine are reports of players managing to deal damage even when they have no ammo. By this we don't mean the feeble swing when you hit the light attack button, but full damage as if you had ammo in it.
Bungie's TWAB didn't address this problem and the next reset will presumably not fix this issue. It's possible that the reason is that this bug is harder to replicate and therefore harder to fix.
Anyway, if you do notice a dip in damage output of your Guillotine, this is the reason.