Did you physically disconnect power? There is stuff kept in memory about HDMI connection, even if they are proxied through a video processor.
In any case the usual chain should be Source -> Amp -> Video Processor -> Display/Projector.
To go deeper into the problem. You carry audio and video on the same HDMI connection. When a format changes a restart of the HDMI Output of your Lumagen may happen and the Marantz doesn’t really know to handle it sometimes. The Lumagen thinks the Marantz is in the correct mode but it isn’t. And the Marantz waits for something. So they are “out of sync” to speak so and think they are in sync. Both devices need to “re-negotiate” their connection, but don’t know that they need to do this.
This is a common problem when you have a longer HDMI device chain for usually video processing. No one really to blame here.
Receiver vendors have ton of workarounds for specific scenarios in their firmware. Guess whatever happens is not covered here.
If you need to have the Lumagen before the Receiver you might try splitting audio and video HDMI intro different ports, run audio to the Receiver and Video to the display/projector. That might help, but can also cause different issues.
I had a similar problem about a decade ago with a DVDO Edge back then. In my case I had to plug a mediacenter into the Edge and not the receiver to solve basically the same issue you were seeing.
Oh boy did I hate the HDMI firmware on that Onkyo Receiver. Awesome device, but the software… And back then I had to find “back channels” for a firmware as well as getting the dongle to update the HDMI firmware. HDMI Firmware updates by customers where not supported. Well times changed as vendors learnt. And shipping that top shelf heavy receiver would have been painful and out of the question (could barely lift it).
The “JTAG Dongle” passed through many hands - someone in Canada created it initially and we passed it on in the AVS forum to the next in line. I think it crossed the globe many times.
Never buying Onkyo again. Currently on some cheap Denon, but using it only as a Preamp and switching center. Makes no sense anymore to buy expensive receivers if connections change every few years (now it’s 4K). Better cheap but only use it as a pre-amp and update when something nice you want integrated comes along.