I'm assuming you already went into the Settings->Display and Sound and changed resolution from automatic to 1080p (you can try both the different frequencies listed - 60Hz 59.94, etc.. ) ?
You have tried unplugging the HDMI cable from the APPLE TV, and plugging the Buzz to it (to rule out the specific HDMI port - since you know the appleTV works higher than 720p) ?
Don't think it will make a diff, but on the X5 SS, under settings->Display and Sound, go into more settings, and enable DLG. Then reboot the box (actually pull the USB-C power cable and plug it back in).
I typically have mine connected to an old Sharp Aquos TV and I have connected it to a few different computer monitors (mostly HP and Dell) and have had no issues with that. It's obviously not detecting the EDID of the TV or misinterpreting it.
As a test, ensure the TV is powered on, and switched to the input in question. Then and only then plug the power into the box. Reason for this test is because I have one older box, that when connected to one of my TVs, the TV is not powered up before the box, then the box actually boots up, but you don't see any video output. Only way to recover that is to either reboot the box, or unplug and replug the HDMI cable (which is a pain considering it is all tucked behind the TV).
I would reach out to them again. RMA may just yield the same issue and just a waste of time. Keep in mind that the X5SS is one of the newest boxes, so it will certainly have a few glitches and combinations (which there are piles off) which they did not consider or specifically test. This can most likely be fixed or patched in firmware if they can figure out why.