IMO heat is just an easy-mode answer. Onkyo's run hot, it's an easy target.
EVERYTHING ELSE in the receiver is almost always OK... but the non-wearable part at the top near the back... likely one of the cooler internal locations... dies from heat all the time. How much heat kills an Onkyo HDMI board??? Why did they only use heat sensitive material there. It's gonna be some pcb with some chips, I'm not personally concerned about heat anyhow.
I would have to guess as well to provide an alternative theory, poor mounting and stress from taught / heavy hdmi cables would be my guess, or just bad QC. Sourced a bad part and bought a few million of em'.