Unit counter doesn't work

Unit counter doesn't work properly, one turn it shows me that I have 300 units and the next turn that I have 150 even if I didn't have any major battles. I noticed it maybe a week ago, and it isn't always wrong, just sometimes. I don't know what causes it, it shows on any maps, any turn etc.

I have experienced this as well, it usually happens to me when I use auto production.

It happens when I don't use auto-production, too.