From the MX300 G2 Admin Guide, pg 109:
Auto: If a second monitor is connected, the layout is distributed on two monitors. Otherwise, everything is displayed on the main monitor.
DualPresentationOnly: All participants in the call are shown on the main monitor. If a presentation is part of the layout, the presentation is shown on the second monitor.
The HDMI output doesn't support mirroring of the main screen, whenever you have an external monitor connected it's meant to expand the system so you can have dual screens, and what would normally get displayed on the single screen would then be distributed across the two like you're seeing.