[June 1, 2016]
The downtime which occurs while your Uptime Monitors are under Maintenance Mode is now visibly separated on your Uptime Reports page:
Furthermore, now the maintenance and non-maintenance downtime is logged in separately if you switch to maintenance more while an outage is in progress. As opposed to before when, if you did this, the entire downtime would have been considered non-maintenance.
You can read the following documentation article which fully explains how Maintenance Mode is affecting your Uptime Reports:
If you experience any issues or problems with this new feature, please open a support ticket and let us know about it.