In my priviledged LXC container, I can only write when I'm root.
Config done on PVE for LXC:
mp1: /mnt/storage,mp=/myfiles
fstab:
/mnt/pve/media/media /mnt/storage fuse.mergerfs allow_other,nonempty,use_ino,cache.files=partial,func.getattr=newest,dropcacheonclose=>
Other configuration details...