Total gameplay duration differences like decreases usually happen due to Steam Cloud Sync issues. For example, if you played offline, Steam wouldn't count your hours. So, during the time you were playing recently, there could've been changes in your internet connection or a general cloud sync issue that lost you those hours.

Thanks. Good info, but I'm not sure it applies here...

I don't see how not counting hours could cause the already counted hours to decrease.

I haven't played offline, anyway. Unless my connection messed up and I didn't notice (unlikely since I'm not the only one using the internet here), but...

The loss wasn't during the time I was playing. I noticed it went down 8 hours at some point within 2 hours during which I was not playing it.

i got the same thing! it showed 400 hours, i synced the cloud, and it suddenly dropped to 247 hours, and i maybe had like 10 hours offline total

This thread was quite old before the recent post, so we're locking it to prevent confusion.