Hey Otrhan,
I managed to get it working, the steps are very strange however.
It specifically only works by doing this:
Simplify using absolute - this then effectively just duplicates the model/textures, rather than reducing tri count.
Clear RC cache - not doing so here will result in the same duplication later.
Simplify that NEW model using relative (using relative first results in the same duplication).
The tri count is only then reduced as it should be.
I tried resetting RC as you suggested but it was still causing the same issues. The only method so far to get things to simplify were the steps above. I haven’t come across this issue before across thousands of other RC projects, so it’s very strange for it to happen here. My only guess is there is something specific in the data itself causing this, what that is however I have no idea.
Thanks for your help regardless!