I double checked the innards of my system and did not find any dust there... Well, it's a pretty new system. The glitch seemed to have vanished too, at least, for some time.

It came back the other day, but this time I noticed a pattern. It would show up if I applied the field defense option to my artillery. It was one particular artillery unit that seemed affected (not the first, not the last) in several battles. I tried different options, clicking the field defense on and off for different combinations of artillery units): the glitch always showed up for that particular unit (all my artillery pieces were horse artillery, btw).

Also, I remembered that in the previous two occasions when I observed this glitch, the AI (not me) had deployed artillery field defenses...