I've discovered at a family member's house that their AppleTV 4K and Gen 3 AppleTVs in family/living rooms on opposite sides of the house sync fine together, but if you try to sync the computer (which is in the same room as the Apple TV 4K) to either one, they're out of sync (computer is ahead). Likewise, she has an Airport Express connected to a Bose Radio in the dining room (in-between the living and family rooms and next to the living room) that syncs fine with the Mac in the family room, but it's out of sync with either AppleTV. Clearly, distance has nothing to do with it as two of these devices are in the same room as each other (Mac and AppleTV 4K) and the Bose/Airpot Express and living room Apple TV are about the same distance from the Mac. It appears the newer AppleTVs (and/or the receiver hardware) are creating a delay iTunes can't account for yet are in perfect sync with each other. Meanwhile, the more direct Mac speakers and straight Airport Express interface are in sync with each other. It kind of ruins the whole house effect....
At my own house, I had perfect sync with iTunes using 1st Gen AppleTVs with a receiver in one room and a pre-amp/amp high-end setup in another room and two Airport Express units in two bedrooms and an AppleTV Gen 2 in an exercise room. ALL of them were in perfect sync. I could walk around the house and everything worked great (muting/turning up/down the volume from the Mac would let you compare as well). So, I don't know why the Gen3/4k units are having issues. I just got a 4K unit the other day at my own house (haven't hooked it up yet) and I have a Gen 3 unit that I just never tested it with since I never assumed there would be an issue and I don't use whole house (party) mode that often anyway, so I'll test it again there and see if it has the same issue or it's the receiver setups here (one is a Yamaha receiver and the other uses an intelligent 5.1 soundbar, although the ATV 4K is run through the TV first due to the receiver not passing the full 4K 60fps + HDR through on the 4-year old receiver. The additional processing might have something to do with it.