No. The projector just turns off. Shuts down exactly as if I were to hit the standby/power button on the projector's remote. I need to hit the power button on the remote to turn the projector back on to re-display the picture. The Pi 5 and receiver continue playback through the process. However, I do notice a brief blip in the audio when this happens.
Hmm. If you asked me to create some code that ran on the Pi and powered off the projector without using CEC (which you say is disabled) or DDC (which the pi will never send without running an external program like ddcutil), I'd say that was impossible.
Your projector is choosing to power off for some reason. It could be an intermittent hardware fault of the projector.
It could dislike something about the signal coming from the pi over hdmi (although switching to standby is very surprising for this - normally you would expect a loss of sync, resulting in a temporary black screen, and recovery shortly after).
The likely causes for it not liking the signal (after it being happy for ~25mins) is something marginal - possibly caused by a poor hdmi cable/adaptor, or an inadequate power supply (the 5V from the power supply does go over the the hdmi cable and powers the display/projector's hotplug/edid logic).
Might be worth timing how long playback lasts before failure. If it is always 25min then that would be interesting (but I suspect it is more likely random).