Comment 9 for bug 1948339

Sorry for the late reply on this issue. I only saw it a few days ago. I've spoken with the Arctica greeter developer and we've been working on a fix.

The issue is this, Arctica Greeter requires a window manager and it invokes Marco, the window manager from MATE Desktop. Marco handles keybindings and by default has a number predefined. Ubuntu MATE adds a few more. This is why you are able to invoke applications bound to keybindings in Marco from Arctica Greeter.

The proposed solution is to add a patch to Marco so that it can be invoked with keybindings disabled and then patch Arctica Greeter to invoke Marco with the argument to disable its keybindings.

I will start preparing patched versions of Marco and Artica Greeter in a PPA for testing/validation.