Welcome to the Ubuntu MATE community, @xirawa!
This is a fairly common issue on Ubuntu MATE (and Xubuntu, actually). I've had this one before. It's likely that this is an adverse effect of a race condition... in UM's case, likely attributed to Arctica Greeter.
Please refer to this thread to replace Arctica with Slick Greeter. Thank you!
PS: Removing Arctica Greeter following above instructions may remove your panel indicators for accessibility and keyboard options. It is advised that you install them back by running sudo apt install ayatana-indicator-a11y ayatana-indicator-keyboard.