Sorry for the delayed reply - super busy at work lately.
So of course now that I've gone to the trouble to post about the problem it's started working again.
The same artist that it was only showing a single picture now rotates through multiple.
But I swear I'm not crazy. The staying on a single picture really was happening, and not for the first time. And glitch with the track timer at the gallery interval seconds really was happening as well. I rebooted for window updates shortly after this post and maybe that cleared something up.
I'll keep an eye on it and post if happens again.