We ran in circles for a couple hours on this one. SOME users (not all, only some) are unable to access the Maps (monitoring) unless they have permission to read ALL possible items in all possible maps. Other users in the same group with the same checked individual permissions of the same user type, are able to see the maps they have access to and not the ones they don't, as expected.
To say another way, if you don't have permission to a host group that is featured on a map, when you go the Maps tab you will not see those maps in the pull down that contain elements you lack permissions to view. You will however see other maps just fine. In some cases however, a given user, with identical permissions etc to one that is working as described, gets nothing when they click Maps other than an error that they have no permissions or the object is missing (something to that effect). Adding said broken user to another group that gives them read on everything fixes this, but then they can see maps we don't want them to see.

I can find no reason why certain users become broken like this, and others keep working fine. I have found no way to make it start working again, only override by opening it up entirely (and trying to limit again breaks that user, but not the other users who just lose the one map not the whole screen). I find absolutely nothing in the zabbix server log about any of this (that log honestly seems to have nothing useful in it that you can't already see in the frontend, haven't found an option to enable more logging...)