Documentation for Maintenance periods states, that:

Tags	If tags are specified, maintenance for the selected hosts will be limited to the problems with the corresponding tags.
In case of multiple tags, they are calculated as follows:

But if a maintenance period is created with Tag, that has not been applied to any problems:

All hosts and problems are still in Maintenance mode. The same thing happens, if the tag is created, but with wrong value (0)

Settings for the trigger:

But problems are still created in maintenance mode: