Drinking a potion of mutation and then shifting to another form via the yuan-ti polymorph options allows access to shapes that I'm pretty certain no player is meant to have access to? I don't even think shifter can turn into most of these.
I suspect it has something to do with the groupings(factions?) that the potion's polymorph options use and how that interacts with the yuan-ti polymorph.
Interactions:
Spider form + Snake morph = Snake (this one works and I assume it's either because spider is in the same grouping or because it's the default form with the potion and not actually broken)
Troll form + Snake morph = Giant
Umberhulk form + Snake morph = Vrock
Fey form + Snake morph = ...leaf woman? (Presumably a dryad?)
Zombie form + Snake morph = Wraith
Steps to reproduce the Bug:
1.) Drink potion of mutation
2.) Use the yuan-ti polymorph (any of the 5 options, all of them have the same outcome)
3.) Discover a brand new world
Expected Result:
The polymorph from the potion is canceled and I am able to hiss to my heart's content.
Actual Result:
Very much not a snake.
Screenshot(s)

Additional Information/Notes Sort of a bonus random bug with this that surely isn't THAT relevant, but all of these forms listed above have their racial type listed as "Yuan-ti Pureblood" despite the fact I'm not sure that tag is EVER used elsewhere? Typically it's human in human form, reptilian in humanoid lizard form, and animal in snake form.