Nope, sorry should have said I rebuilt it. The first MD was rebuilt as part of that dist-upgrade issue. Had the same dist-upgrade issue with the MD I'm talking about now, but that's neither here nor there, I wanted to rebuild it anyway to see if it sorted the volume issue, and it didn't. To summarise:
MD without upgrade or rebuild - no volume issue.
MD upgraded (with those pluto errors), no rebuild - has the volume issue.
Rebuilt and re-upgraded (solving pluto errors) - has the volume issue.
Consistent with both MDs.
FYI I have another MD which is identical hardware-wise to the first one, which I haven't yet upgraded and doesn't have the issue, just in case it's useful for any troubleshooting.