Entries are somehow resetting their status to "Closed" soon after editors open them

The writers and editors at our magazine have been complaining that EE seems to be switching entries that have been opened for publication back to a closed status.

Has anyone any thoughts why this could be happening? I’ve had five reports of this in the last month. It was happening before the update from 3.4.3 to 3.5.2, according to reports. We’ve never seen anything like that before in our 9 years of using EE.

Terry