On Fri, 09 Jul 2021, Graham Cobb wrote:

quoted

How/where can I escalate it further?

Try complaining to NFS. It might be that it would work better if NFS
assigned different NFS filesystem IDs to each subvolume - I don't know.

Better than complaining...:

Apply the patch you can find at

 https://lore.kernel.org/linux-nfs/162457725899.28671.14092003979067994699@noble.neil.brown.name/T/#mc4752a019af79cbb166d5338d6ed0db141832546 (local)

then apply the fix described at

 https://lore.kernel.org/linux-nfs/162457725899.28671.14092003979067994699@noble.neil.brown.name/T/#mc26984e10e7837e28aca3209fcb03b38a4df6fe7 (local)

which I think is shown in more detail in a subsequent message in the
thread.

Then confirm for yourself that it works.
Then reply to that thread (or send a new message to linux-nfs) saying something like:

 Hi,
  I've been having problems with NFS and btrfs too.  I found this patch
  and it works really well for me.  Any chance we can get it included
  upstream? 

That might spur us on to further action - enthusiasm is much better than
complaints :-)

(the problem is not that NFS doesn't assign different filesystem IDs,
 the problem is that NFSd doesn't tell NFS that there are different volumes).

NeilBrown