That rings a bell:
When looking at some logs I got the feeling that there might be some GNOME’isms in it the SPICE agent. Or perhaps the switch to Wayland for GNOME and KDE has something to do with it. Not a Mate user, but I believe it’s still X11 only, like LxQt.
I sometimes get the same in GNOME. I can get it back by setting the display resolution from the drop-down list to the one of the VM windows; should be easy to spot, because it’s usually not an exact display resolution, e.g. 1864x724. After that the VM starts responding again to the resize events. Also, setting the display to the host’s native resolution and the refresh rate to 75 Hz, when the VM is in fullscreen mode, seems to have the same effect.