User login fails silently when maintenance mode is active

See the screen capture with the current behavior.

Summoned the UX team because I would like to ask their usability review on the original behavior and a suggestion for changing this.

We made interesting findings with @rkoller on Drupal Slack and maybe we were seeing ghost or 🐛 Inform users that the maintenance mode message is not displayed on cached pages Needs review (or similar) made the maintenance page disappear and only appear after cache clear for the user login form submit.

https://drupal.slack.com/archives/C1AFW2ZPD/p1722505121970489

I am going to keep this issue open, we will get back to this later. Untagged UX for now.

... and adding a dump of the Slack conversation.

In this case, we have to show some message to user the user about their permission. Because the user account credentials are valid and also session generating and closing on the time.

So adding some valid message on an attempt in login will give an idea to users. This scenario will be understood by the technical people. But the Content editor and normal users having an account on the Drupal website site will not understand.

Suggestion 1: Warning message on Login page
On the Login page, we can show the default message as "The login is allowed only for the restricted permission users."

Suggestion 2: Warning message after validating login
In this case, we will validate the login, If the user has bypass permission will log in to the website. For other users, we are showing the maintenance page. On the top of the page, we can able to show the message.

Already we are facing a cache issue on the maintenance mode on [2804387]. So showing on login page will be better.

Seeking suggestions from others.

https://www.drupal.org/project/maintenance_message → implemented something similar as Suggestion 1 for Drupal 7.

I would make the message configurable, otherwise I like the current state of the Draft MR.