On some occasions, HDMI backpower from your monitor causes power-on issues for your MEGA65.
A workaround available for now is to place a HDMI-splitter between your MEGA65’s HDMI port and your monitor.
Here are some HDMI splitters suggested by the community:
Canada Market:
JudgeDrokk for Uk:
A few mentioned on Dan’s welcome guide:
Mickoz for Australia: