Hello,

Your problem is not clear. When a site is in a maintenance mode, it does not show a login form to log in as any user and the only view of the site is the Coming soon page.

Now I have tested at my end, I was logged in as a customer, enabled the maintenance mode and try to go to the shop page I was redirected to the coming soon page.

Make sure you have enabled the mode in the Theme Settings > Maintenance mode not by a third party plugin.

Best Regards