Ah, ACLs, had the pleasure of working with these again last weeks.

It gets really curious when even the Arch wiki doesn't really know what's going on (talking about mask and effective permissions):

The factual accuracy of this article or section is disputed.

Reason: The original note about the --mask option (which was taken from setfacl(1)) was determined as inaccurate, but the new note does not seem correct either. See the talk page for details.

From trying, I can confirm that the info presented further down is wrong.

Once you read what it actually does and why it's the way it is, it makes more sense - not that I remember it now - but at least there was a coherent design decision behind it