Another theoretical explanation: your tinnitus is always there, but you're not normally in extremely quiet places, and the quiet from the ANC lets you notice it better.
No, I was definitely not hearing high pitch when noise cancelling was on. I was hearing it when I removed the headset after using it of a few hours. It usually disappeared the next day. One time though i could still hear the high pitch for several days, that’s when I decided to stop using the QC25 and the high pitch eventually faded away and never came back, until AirPods.