For those of you who are also testing Ubuntu-MATE 24.04 LTS pre-alpha:
The reason that the graphical login is not working at the moment is because of work that has to be done on arctica-greeter. Changing arctica-greeter temporarily to slick-greeter will give you the graphical login back:
sudo apt purge arctica-greeter
sudo apt install slick-greeter
When arctica-greeter is ok again you can change it back:
sudo apt purge slick-greeter
sudo apt install arctica-greeter