In case you are not already aware of this:
When you use the cool new scheduled Maintenance feature in OpsMgr 2016 for an object, it will overwrite any existing Maintenance duration for this object!
Let me show you a brief example:
In my lab I will place a server into a longer maintenance duration for several days (e.g. for a long term hardware maintenance):
Now imagine that somebody creates a shorter, scheduled Maintenance (in my exmple: 15 min) for this particular server (directly or via a group), e.g. for a weekly Maintenance window :
Once the scheduled maintenance task has been running...
... the maintenance window on our server has been reset to the time frame of the scheduled maintenance task and the original duration has been overwritten:
Please keep in mind:
The scheduled Maintenance mode will NOT check if an object is already in Maintenance. It simply overwrites the existing Maintenance duration with its own duration.