Comment # 4 on bug 1190272 from Franck Bui

(In reply to Linus Kardell from comment #3)
> Maybe. There is a "systemd[1]: Reloading" shortly before it starts
> unmounting stuff.

If it's not triggered by the btrfsmaintenance units then it would be
interesting to figure out which unit does that.

Looking at the logs, that might be due to grub2-once.service especially since
you said that the problem seems to occurs after hibernating.

Could you try to modify /usr/lib/systemd/system/grub2-once.service by replacing

  ExecStartPost=-/usr/bin/systemctl disable grub2-once.service

with

  ExecStartPost=-/usr/bin/systemctl --no-reload disable grub2-once.service

ie add the '--no-reload' option ?