Playtime recording is not working correctly. Some time is being lost.
I've seen this happen twice: I just played a role for slightly over an hour and maybe about 1/3 of my time was lost. I saw this happen a few months ago when trying to increase my playtime on said role too, so I can safely say there's an issue.
I don't have any exact numbers and I'm not sure if there's some condition that causes playtime to not count.