Not sure how you guys manage to change grub to try to load a snapshot instead. Also, grub on rpi?
Sounds like /boot/cmdline.txt gets changed. I first said urbackup has nothing to do with it, but you are the second person I have seen reporting this. Maybe there is something to it.
Check this thread out for a probable solution since you say you use grub.
Edit
I realize now maybe you are talking about grub on your computer that backs up TO the rpi urbackup?
Still really confusing how your grub change to try to run off a snapshot instead.
Never the less, the link above should help you figure it out. As long as you have not REMOVED anything from your computer devices/partitions, the data should all be there. Creating a btrfs snapshot would not remove anything (unless you removed partitions).
If you provide a little more info on your setup I might be able to help further (I really understand the feeling of loosing data, but I feel you probably haven’t lost it, it is just not mounted correctly)
You CAN run off a btrfs snapshot, but grub has to be able to use btrfs AND the mounting in f ex fstab (or systemd if you are using that) is a little different).
Let me know and maybe I can help you out.