0 If both the HDMI MONITOR1 and HDMI MONITOR2 connectors are
connected and “Resolution” is set to “Auto”, the signals are output with a
resolution that is compatible with both TVs. When “Resolution” is not set to
“Auto”, video may not be output. In this case, set a resolution that is
compatible with both TVs. (v p. 201)
You can check which resolutions are compatible with your TV in “HDMI
Monitor 1” and “HDMI Monitor 2”. (v p. 260)
0 This can be set by pressing HDMI OUT on the main unit.
Looks to be that this could be the root cause of the behaviour. Perhaps it helps, if you power off the TV while using the projector?
My reading of the Marantz manual is that if you want to display the same content on both the TV and PJ at the same time there is no way it can output 2 different resolutions. @angry.sardine’s solution would be your only option. But you won’t get HDR converted to SDR for the TV.
If you want to use one display or the other but not both at once the AVR should, logically, support 4k or 1080p depending on which monitor output you select. The note on page 201 is rather confusing - why should video not be output?