I don’t understand the sense of a “Join as UVs” function if it’s so sensitive and easily produces messy results
The Join as UVs tool is like copying UVs from one mesh to another. It requires that the meshes have identical mesh data (vertex count and vertex order). It should be a pretty black-and-white tool, meaning it either works right or it doesn’t; pass or fail. So I’m not sure how it could give you messy, kind of working results 🤔
However this theme of slightly inconsistent mesh data is in the same category as your mirror shape key issue. Somehow you’re tweaking the mesh data just enough to mess up these functions that depend on perfect mesh data. This can be hard to pinpoint what exactly caused the issue until your awareness in the moment realizes it. My best advice is just to be very mindful of the tweaks you make every step of the way because things get unstable if you don’t.
Everyone developing advanced skills has to learn this the hard way, including me. Modeling in the beginning is fun and carefree, especially sculpting. But when we get into the technical weeds of animation production it’s imperative that we’re careful with everything. Every production I’ve been a part of is a LOT of technical problem solving like this. There’s just so many little things that can go wrong / quirky along the way.
The good news is that you solved it yourself. Often re-doing the problematic step is the way to go 👍
