Suspend button not locking screen on 21.04

Since upgrading to 21.04 MATE, I've noticed that intermittently the screen-lock doesn't always come on when you press the suspend button.

I'm running a desktop with power on all the time. It is the same whether I press Suspend through the menu or use the lockscreen shortcut.

I have been upgrading MATE continuously since 18.04 and never encountered this issue before.