It looks like the fine behaviour described here, 8 years ago, has reared its ugly head again.
I’m not sure what the cause is, might be Tahoe, might be the latest Tahoe.
The fix, user-side, is to open terminal,
sudo nano /etc/nsmb.conf
Change protocol_vers_map to 7 (instead of, IIRC, 4)
And then use cifs//[ROCK] to access your files.
Let’s hope this one slips through the cracks of the growing neglect and lack of care the Harman buyout seems to have spawned.