Finally just realised why my imposters kept failing, here's what I found.

An avatar with a PC and android version uploaded generate imposters just fine. If you attempt to generate an imposter for an avatar with a PC, android and iOS version, a failure message will be displayed in game after a few minutes.

I was unaware android avatars would auto convert and display on iOS, so was uploading an iOS avatar identical to the android one. Doing this broke imposter generation. Deleting and re-uploading the avatar without the iOS version successfully generated imposters.

I kinda think this is not supposed to happen so here we are