I had buffer issues as well:
Hetzner server location at that point was in Finland. I made a topic in here without any solution .
I newly bought a new server in Hetzner Germany, and then my problems was solved:
Running with :
mergerfs version: 2.28.1
FUSE library version: 2.9.7-mergerfs_2.28.0
fusermount version: 2.9.4
using FUSE kernel interface version 7.29