Just a sidenote, I circumvented this issue, without UID mapping.
As stated in OP, I just wanted to share Bind mounts between containers, without the possibility that files are unavailable or read only to some containers. This doesn’t necessitate specific users, just preset rights. So I added 777 ACL-s to the root folders on the Host - it’s not a problem security wise, as containers only get the bind mounts they should have full access to.
For anyone with the same problem, think about if you are truly needing a specific user to own the files - if not, ACL is a good, and much simpler solution. I still wish I knew what the issue was with the UID mapping… it might still be possible that a future application I add might need specific file owners. I’d still wish to know what’s wrong with the above config… I hope I don’t have to do it the hard way.