I don't think I did, but I spent a lot of hours troubleshooting and basically every time that container went through /mnt/user, shfs crashed. If anyone wants to test it, feel welcome to do it in my place. I don't have time to further troubleshoot this in the coming days. Just sharing what I found. Sorry
After a lot of searching, I gave up. Shfs completely crashes when restarting or stopping the container (95% of the time, sometimes it survives) Temporary workaround: map all the stuff to /mnt/cache/appdata/unifi-server-os instead of /mnt/user/appdata/unifi-server-os.