Anecdotal but I have experienced similar with QC35s. Wear them for a bit for meetings etc (not loud) and tinnitus is noticeable when in bed that evening. Don't use them for a few days and tinnitus is not noticeable. It certainly feels like there is a definite correlation.

I assume the ANC is still pumping in a lot of frequencies that do not get 100% cancelled-out, but are outside of my hearing range so I do not notice them in the "silence" but which is hammering my physical ear bits with audio energy.