Hello @Roach and welcome back to the community!
Here I have to guess, because I don’t animate, but I guess that your transformation mask introduces partial transparency into your animation.
And if that is true, then your issue is that you are using the wrong format, GIF does not support partial transparency/partial alpha-channel.
You need to look for another format that supports partial transparency/alpha.
Hopefully an animator from the community can confirm this, but I’m very sure it will be the culprit.
Michelist