I installed Mageia6 dev alpha1 to hard drive, then proceeded to add all 1149 updates, so I was up to date.
I also created a second login user.
Logout and Login worked okay for user1. User2 was not accessed yet.
With user1, I added the Mageia control centre, and also added Firefox to the toolbar/taskbar.
Logout was okay.
Login showed black screen and only the network monitor running. Moving the monitor back on the screen created ghost panels. There was no access to other applications. Pressing ALT-TAB showed only the one application running.
Logout was by pressing the power button.
On reboot, and login again, I returned to the same black screen and only the network monitor running. ALT-TAB only showed this one application running, and shutting-off the application, eventually a new network monitor application began, each time I shut down the application, it would restart. Logout was done using CTL-ALT-DEL

Login into user2, I had a normal MGA6dev1 desktop. I created user3 to see if this is repeatable. Logout, login, no problem. Login, add an application (Firefox) to the toolbar still okay until I logged-out. on login, I entered a black screen with the only application running being the Mageia welcome splashscreen. Logout with CTL-ALT-DEL, and login, again only showing Mageia welcome splashscreen application, with no other applications running (tried ALT-TAB).
Logout using CTL-ALT-DEL, login into user3, and arrived at normal MGA6 dev1 alpha screen.
Right now, writing this bug report from user3 login (user1, user2 both have black screen on login).