Intermittent HDMI handshake failure. When source, AVR and TV all wake at once, the negotiation between them (HDCP authentication and EDID, the TV telling the chain what it can display) occasionally completes wrong, and a power cycle forces a clean renegotiation. It is not a fault as such, it is a race, and you can make it rare or gone.
The order I try:
Power-on order. TV first, then AVR, then source, with a beat between each, so each device sees a stable partner when it asks. If CEC turns everything on simultaneously with one button, that convenience itself can be the trigger; try disabling CEC for a week and see if the blank starts vanish. Some AVRs also have an HDMI standby pass-through setting that changes how the chain wakes; toggling it is worth a try.
The cable. Replace the suspect HDMI cable, especially the AVR-to-TV run, which carries the most demanding signal. Marginal cables produce exactly this some-days behaviour, fine at 1080p and flaky at 4K HDR when the morning is humid. A certified cable of the right length is not expensive and settles the question.
Firmware on the AVR and the TV. Handshake fixes are the commonest item in release notes, because the manufacturers keep tripping over each other's implementations. Update both, on stable power.
Signal format. If the AVR has a per-input HDMI signal-format setting (enhanced/standard, or a bandwidth setting), match it to what the source actually sends. A source pushing 4K 60 4:4:4 into an input set to standard will handshake sometimes and not others.
None of that helps and it is getting more frequent over months? A degrading HDMI board announces itself this way, blank starts becoming blank evenings. That is an authorised-service conversation, and better had while the unit is still under warranty than after.
Tell us the TV, AVR and source models and which cable runs where, and we can usually say which of the four it is.
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