HDMI only works directly through the TV, doesn't work through A/V receiver

As described in the title, I can boot normally and see everything just fine when I'm going through my television's HDMI port. If I try to do the same through my TV setup's A/V receiver I get no audio or video. Just a black screen. No BIOS or anything. Has anyone experienced and/or fixed this?
Comments
HDMI works when connected directly to my Dell monitor, but does not work through an ATEN CS682 KVM switch.
None of my PCs/laptops have this issue.
The UP Squared (Celeron N3350) is the only one that causes issues, and fails to display any video by default. It works OK when connected directly to my TV. It fails to work at all by default if connected via a Denon AVR. However if I boot up with the UP Squared connected to the TV and then plug in to the Denon AVR it does work.. The same is true whether I use HDMI or an Active Displayport to HDMI 2.0 converter.
I HAVE got it working in LibreElec by forcing EDID (I extracted a working EDID on a Haswell Celeron box and force the UP Squared to use this) It looks to me like the EDID interrogation with some devices is failing - or not happening quickly enough.
Definitely an issue that needs investigating - particularly as it happens in both Windows and Linux.