I'm sure these were behaving normally just yesterday. There was a stack of 10 10/10/10 and a stack of 5 10/10/10 which I merged. I think I checked and they were ok.
Now they're 10/3/10/3/10/3. If I split, they go 3/3/3, put them back and it's back to the 10/3/10/3/10/3