APPLE TV

In Apple TV settings > Remotes and Devices> Control TVs and Receivers , click it to turn it off.

Having it on seems to interfere with other equipment.

HDCP 2.2 is a security measure (encryption), mostly known for 4K content over HDMI (but not limited to there). It must be supported on all devices in the chain: Apple TV, receiver (video passthrough/repeater), and TV.

HDCP 2.2 over HDMI requires a HDMI 2.0 port (but you can have a HDMI 2.0 port without HDCP 2.2); the required cable depends on the bandwidth needs (up to 18 Gbps: ‘premium high speed’), not the HDCP version.

The 4K passthrough with HDCP 2.2 support primarily has to do with the introduction date of the model. It was not that common in 2014 (when the STR-DH550 was introduced), also the year that the first equipment with it came on the market. You should still check the features list with new equipment, though.

I take it you Apple TV 4K is connected to your TV, not the receiver, and the receiver and TV are connected to each other on the ARC ports? All HDMI (no splitters)?

That receiver does support 4K passthrough, but not HDCP 2.2, which almost all commercial 4K content will require, so you can’t use the passthrough in a practical way. That shouldn’t be affected by volume changes, though, AFAIK.

There are too many to list now, in 2018. The link in my first reply had 11 Sony 4K AV receivers with HDMI 2.0 and HDCP 2.2, and 5 4K AV receivers that didn’t.