Shutting down the thumbnail and nuking the thumbnailer cache didn't help.
Following that I restarted unity8-dash, and it started behaving again.
Therefore I would surmise that the problem is either in the actual thumbnailer QML plugin, or perhaps in the built-in QML image cache (not really sure how they both interact).
I'm about to try restoring the thumbnailer cache to see if it causes the trouble again.
Shutting down the thumbnail and nuking the thumbnailer cache didn't help.
Following that I restarted unity8-dash, and it started behaving again.
Therefore I would surmise that the problem is either in the actual thumbnailer QML plugin, or perhaps in the built-in QML image cache (not really sure how they both interact).
I'm about to try restoring the thumbnailer cache to see if it causes the trouble again.