Evolved an Ekans and it turned into an arbok with the Purple/Black textureless colouration. I guessed it was because Ekans doesn't have any Forms but then evolves into Arbok who has forms, and it doesn't know what texture to apply to it immediately during the evolution animation. I tested it with pokemon that have both forms, such as Deerling into Sawsbuck and it worked fine because Deerling already has the determined . I then tried Spewpa evolving into Vivillon as it also gains a form similar to Arbok, and Vivillon also had the Purple/Black texture, so I assume that's what it is. This bug fixes when you put it back into the ball and throw it back out.

Steps to reproduce:
Evolve a Pokemon such as Ekans or Spewpa who do not have forms, but evolve into a pokemon with forms.

Version: pixelmon-1.12.2-8.3.3-universal

Hi thank you for reporting this bug as I found the same result as you. When evolving certain pokemon they get really buggy textures for reasons that you assumed. Pokemon like Burmy, Ekans, and spewpa get really buggy textures when evolving. I will give a full list of the ones I found below but each have a split evolution or a multi form parent.

Pokemon we found
Alcremie
Mothim
Lycanroc ( won't even be the right form if midnight form)
Vivillion
Arbok
https://imgur.com/a/adFuYmW

Moved to internal.

Should be working in 8.3.5, please reopen a ticket if its not the case,
Thanks for the report!

Still an issue in 8.3.6, the issue is open on internal.

Recent ticket: tracker.php?p=2&t=16921