I can tell you that with my 7.2.2 I don't have any dialog bleeding. And all amps/channels are in use as the front speakers are bi amped.
I'm using 7.1.4 at the moment and I happened to notice some dialogue on the surrounds yesterday. Though I was watching live sports - the Formula One race, it was in Atmos I think. I assumed it was a live mixing thing - and the voices on the surrounds were pretty quiet anyway.
Anyway, I set up two Scenes today so I could rapidly switch between 7.1.4 and the other 5.1.4.
I played a Netflix episode with an Atmos soundtrack and could hear no vocal bleed in either setting. Same with a 5.1 soundtrack - no bleed. I used Dolby for both tests.
A programme with a 2.0 track - the surrounds had the dialogue in both 7.1.4 and 5.1.4. I then switched to Surround AI and the surrounds fell silent.
I don't know what Dolby is supposed to do with 2.0 sources. I'd assume that Dolby Labs tested Yamaha's implementation to check it was correct.
p.s. I did a similar 2.0 test on my A2080 system and it was the reverse - I got no dialogue on the surrounds with Dolby - but they sprang into life when I switched on Surround AI. Very odd.