AirPlay 2 - ATV and HomePods out of sync

I have extensively tested AirPlay 2 on my iPhone 8 running iOS 11.4 and I am encountering delays/sync issues when playing back audio to HomePods and ATVs simultaneously. Here's what's happening:

** Using iTunes on a MacBook Pro to stream music to two HomePods, a 4K ATV plugged directly into a TV via HDMI and a 1080p ATV plugged into a stereo receiver via HDMI - all running current OSes (this is the original AirPlay setup I've used for years): Music plays fine with all four devices in sync.

**Using Music app on iPhone 8 to stream music to two HomePods, a 4K ATV plugged directly into a TV via HDMI and a 1080p ATV plugged into a stereo receiver via HDMI (AirPlay 2): Music is in sync on both HomePods, but way out of sync on both ATVs. The amount of it being out of sync varies, but it's never been completely in sync like with AirPlay 1 via iTunes. Adding/removing ATVs via the Control Center menu occasionally makes the sync a bit better, but it's all just degrees of unlistenable.

I've seen some posts around observing similar issues and some theorize that there's latency introduced with the HDMI connection and that's causing the issue. This clearly isn't the case since using the same 4 devices via iTunes works great. (That also begs the question: Is AirPlay 1 compensating for this latency and AirPlay 2 isn't? If not, how is this an "upgrade"?)

SO - why isn't AirPlay 2 able to play in sync across the only three supported AirPlay 2 devices in current existence?