The original poster said it may be related to “Restore from backup”, however it is unrelated as i have the same issue without ever having restored from backup.
For those that have the same issue, has anyone noticed that it maybe happens to maps with props outside of render distance, which maybe causes saving issues due to the low Creative 1.0 render distance?
What i have noticed though, is if you back out of your map, while still in creative (the creative lobby with the rift), then change to a different map and then back to the other map, it seems to reset the ~30-60 minute timer until the bug happens, at least from my experience it seems like it as i constantly jump back and forward and only notice the bug after being in the map for a long time.
Edit: The bug also does not happen every single time after ~30-60 minutes.