Left ear fluttering with ANC on. its like driving with the car window slightly open.

That certainly sounds like they aren’t functioning properly. I suggest you contact Sonos support and see what they have to say. Defective new headphones would be covered under warranty. They should make it right.

But first, I would try to reboot the headphones and see if that helps.

https://support.sonos.com/en-us/article/reboot-sonos-products

I agree. Just hope there's a way they can resolve the matter quickly and without cost. Yet another hassle that modern life can do without though. 

I feel like a sucker after so many issues with my home Sonos system; I hoped they had finally resolved the teething troubles with the headphones. 

It's very disappointing and my flight without ANC was a cacophony of babies and hubbub. 

I should also add that another symptom is that the left ear also emits a sort of white noise and pressure in aware mode. 

Compared to my Sony xm5 the Ace's ANC (before I updated the firmware) was not near half as good.  But the sound quality I would argue is better than the Sony for my musical tastes. 

And I suppose for Sonos tech it's worth emphasizing that when I first tried the headphones, they worked just fine. This issue only arose after updating the firmware. 

Yes, I would let them know that.

Try a factory reset of only the headphones, run an update check. Reboot and give it another try.