Hello There, Is it expected to restrict wp-admin activities when switched on Maintenance mode?
Getting below message "Briefly unavailable for scheduled maintenance. Check back in a minute."
That's the Maintenance Mode activated from your panel, using WordPress functionality so the answer is yes.
ok but why would a user activate Maintenance mode if WordPress admin functionality is prohibited?
Any use cases, except backup restore?
If you change the Maintenance mode from Websites - Manage it will go away.
Or you can delete the file .maintenance if you prefer.