Logs are fully wiped after reboot as these are stored in RAM.
If you find your TRB140 with less uptime than before - it definitely reboots itself as it should.
If you want to save the log right before the reboot, you could do scheduled reboots yourself with the crontab.
Here you will find enough information to configure it:
So basically what you want to do is to set a crontab for the device to save logs from "logread" to /etc/somekindoflog.txt just before the crontab of reboot execution.
Command for copying the logs: logread > /etc/log.txt