Roon --> AVR --> Projector / HDMI outputs reset whenever I turn on/off projector

I’m having very similar issues over here. I tend to believe that this is not a Roon-specific issue, but more likely something inconvenient in the HDMI protocols in genereral.
(HDMI can be a b*tch when source and ‘endpoint’ are not constant all the time…)

I’m by no means an expert on this. But from what I understand, HDMI is always doing some negotiation (‘handshaking’),between source and ‘receiver’ (‘sink’) of the signal.
During this handshaking, they tell each other of their audio and video capabilities. (For instance : I can process up to 96kHz, 24bit, 1080P, 60Hz, 3D, etc).
This handshaking is done, using information called ‘EDID’.

Your receiver stand-alone, might have some different A/V capabilities, than your receiver-coupled-to-your-TV-(or-projector). For instance : it might only play stereo in the latter case, or, it might only do 720p, while it does 7.1 and 1080p as stand-alone (I don’t know, it’s just an example).

In that sense, it seems quite logical that your endpoint ‘disappears’, dependent of the power state of your TV.
The capabilities of the ‘sink’ or ‘endpoint’ really do differ, depending if the TV is ON or OFF, and I feel they really should be treated as different devices (as they ARE different).

It’s not exactly cheap, but I know there are devices that can ‘copy’ the EDID information of a (chain of) device(s), and constantly present that information to the source.
Even when changes are made in that chain. Examples are ‘Gefen HDMI Detective’, “Dr. HDMI”, and probably many others.

I’ve ordered a Dr. HDMI today, and I’ll try to keep you informed how that works out. Will probably take longer than a week or two.

Other than such a device, I doubt that Roon could implement a solution to your issue :(. Seems to be ‘intended behaviour’ of the HDMI protocol, to me…