tldr: It's ok, no possible data corruption.

Asked at the mailing list too, and they explained that the subvol UUID
is just used a sanity check for btrfs send and btrfs receive.

...
The UUIDs on subvols are only really used internally to that filesystem, so the kernel doesn't have a chance to get confused. The main thing that could be confused is send/receive, but that's a matter of possibly losing some validation (thus allowing you to do something that will fail) rather than causing active damage, as in the duplicate-FS-UUID case.
...

from http://thread.gmane.org/gmane.comp.file-systems.btrfs/50909/focus=50917

Now I can sleep better :p

tldr: It's ok, no possible data corruption.

Asked at the mailing list too, and they explained that the subvol UUID
is just used a sanity check for btrfs send and btrfs receive.

...
The UUIDs on subvols are only really used internally to that filesystem, so the kernel doesn't have a chance to get confused. The main thing that could be confused is send/receive, but that's a matter of possibly losing some validation (thus allowing you to do something that will fail) rather than causing active damage, as in the duplicate-FS-UUID case.
...

from https://www.mail-archive.com/[email protected]/msg49133.html (was http://thread.gmane.org/gmane.comp.file-systems.btrfs/50909/focus=50917)

Now I can sleep better :p