Slitherine

As you can see in the screenshots below, between one turn and the next, my army lost a huge chunk of experience, with no notification. These units were produced over many turns, with some only recently joining the army, and some having been there since the start of the game. If there is another explanation than a bug, I would be grateful to know it.

Maybe you lost military expertise along the way; unless I'm also under an unidentified bug (or don't understand a mechanic), I've noticed more than once all my troops losing experience as my expertise would go down, especially during early game when expanding with a nation that started on high expertise and above.

That was the only non-bugged explanation I could think of, but it doesn't make sense to me. As I said, the units were recruited at various different points, they had earned different amounts of experience. It doesn't make sense for them all to lose their experience at the same time. If this was an intended feature, I'd hope for at least a notification, as the loss of that much experience could easily turn a victory into a defeat.

In addition, the cavalry have been reduced to less experience than they were recruited with, which makes no sense whatsoever.

Did they lose experience(xp) or experience levels(Stars) or both?

Just checked this, they lost stars/levels, but their number of experience points remains the same. So the Spartan Hoplites, for example now have three units listed as: level 2 with 75 exp, one unit: level 2 with 74 exp, and one unit: level 2 with 47 exp. It is a similar situation for the other units.

Does this help to explain what has happened?

I think they should regain their levels next turn but honestly I have no idea what happened. If you switch commanders then that would lower their levels for a turn but I can see that you didn't.