Session Manager has code which compares the flags of the current process to the flags stored in Profile Prefs, and if it detects a mismatch - initiates a reboot (see NeedRestartToApplyPerSessionFlags() in user_session_manager.cc).

This doesn't work properly in Mustash: the flags stored in Profile Prefs contain the --mash flag, while the current process is the content_browser process, and it is started without the --mash flag. One possible fix would be to compare the flags stored in Profile Prefs against the flags which were used to start the root Mustash process.