Indeed, in release 31.3 there might be some cases when the logs are being saved with the wrong date in the interface. However, they shouldn’t just disappear. They still should be available but maybe under the wrong date.
Do you have the name of the log that got lost after the reboot? If you do, please send the Full system report and the log’s name to support@emlid.com. We’ll look into it.
I see that my colleague Zoltan has already answered you in this thread. Indeed, the fix for all the Reach devices was released in 31.7 firmware version. Please let us know if your issue persists after the update.