Like many others, I have had problems getting the screen to lock on returning from suspending my laptop. There are many threads here and bug reports (just search for "suspend lock").
In my case, I managed to solve the problem by changing two variables using dconf editor:
org.mate.power-manager.lock-use-screensaver - false
org.mate.screensaver.lock-delay - 0
I'm not sure which of these did the trick but I offer the information to others who might be struggling.
I'm running 20.04.4 LTS (Focal Fossa) 64-bit with MATE 1.24.0, all fully updated as of 19.5.2022.