WetGeek And if it turns out to be yet something else, Solus is pretty quick to reinstall.
As it happens, that's what I ended up doing after breakfast. Everything still looked fine, so I figured it would be a lot faster to reinstall than to spend more time trying to figure out where the problem was. There's still a bit more configuration to do, but that won't take much longer.
It also provided me with a fresh opportunity on this workstation to change some things that I've been putting off. For example, I changed my swap partition from 20 GiB to 48 GiB. That might seem excessive, but this workstation has 32 GB of RAM, and I want to be able to hibernate safely, even if some of the swap is alreay in use at the time.
I know the chances of that are vanishingly near zero, but this Optiplex came with a 1.0 TB NVMe SSD, and with the NAS available to do the heavy storage, I only need about 120 GB of that for the system. There's an embarrassing excess of disk space I'll never use, so I figured, why scrimp on swap space?
Again, thanks. Your answer was a good lead, and I learned a bunch about where Solus puts things in the system. I just couldn't find anything wrong no matter where I looked. I figure life's too short to keep that up.