Create maintenance (m1) with data collection for host group with tag filtering
It works as expected
Create maintenance (m2) with data collection for same host group (or nested hg) with another tag filtering
There will be just first (m1) maintenance period active (and visible!)

Only one maintenance can be assigned for now. Nodata maintenance takes precendence. Other things being equal, the first one created will be active
In large organizations where different people (teams? departments?) can create a maintenance - this can be a problem.