Got it in one, thanks Zoki. What that was, was the shared file for my docker config dataset, that used to reside on a pair of SSDs, both of which failed at roughly the same time. I pulled the dead drives and restored from backup, and then just did a symlink from the old dataset to the new backup and left it at that. That was like 6 months ago too, so clearly I haven't made any Samba changes since then.
Thanks for the fast response, appreciate it.