The usual behavior of Milliman Mind is to kill the session after 3 minutes without an open web browser on the project. Unfortunately, it could occur during a run if the computer is going in sleep mode.

You can now go to Settings > Other > Click on the Switch to keep my session active 48h.
Then your session will be kept alive for 48 hours, giving you the possibility to come back to your project after your computer goes to sleep. Please note that you should not close the open tab with the Milliman Mind project.