I don't use my gear like that, but for the purpose of this exercise I tried to reproduce the issue.
I see the same thing when I kill the receiver (HTPC and TV remain ON). When I power back the receiver the image is stretched, etc to a very strange degree and I have to exit and re-enter Theater View (normal, Windows interface stays normal). Ben, my case is not like your screenshot above, the image is bigger than the screen, like 4-6 times bigger and I get to see only a magnified fraction. It doesn't happen if I turn the TV Off/back On or if I put the computer to sleep and wake it up. But however I haven't tried all the possible combinations.
ATI 4600 (Catalyst 9.11) - Onkyo 606 - Sony TV. All connections on HDMI, no DVI to HDMI, no hdmi splitters, etc.
Note: the results vary if I actually play something when I turn off the receiver. When I turn it back on, I can see it struggling a bit with the handshake, the image and sound are on for a few seconds with heavy stuttering, then the file stops playing the I'm returned to the Theater View interface... which now has all the correct aspect.