The Maintenance page is not documented very well. When you create a maintenance window, I think a lot of people assume that it will silence Actions during that window (given that the Actions are set properly). But that is not the case, you also need to add a Maintenance Period for that to happen.
In the page that comes up when you click "Create maintenance period", it would be helpful if the 2 sections weren't both labeled "Maintenance". I would suggest that one be called "Maintenance Window" and "Maintenance Period" (or something to differentiate them), and make it clear that NOTHING will happen unless you include "Maintenance Period". Ideally it would not let you save unless both are filled out.
I've had to explain the many times to our zabbix users. It seems like it would be a simple fix