As I am aware, I've read through all of my mods and I cannot find out what modifies mob data in a way that presents itself as unique and different.

I don't know what is making it happen, but all vanilla (specifically hostile) creatures are added as a new morph.
My theory is that it was potioncore, I just need to know what mod it is so that I could either toy with the config or remove it entirely.

As Morph has just been rewritten and released for MC 1.16.5, I am clearing the issue tracker as I have not provided any support Morph for 1.12.2 and earlier. My apologies for not addressing your issue.

My issue was potioncore.

I don't really know a workaround with it's specific data, but I would remove potioncore if you are sick of this morph issue.