This is one the most popular topics in the Sony vs Bose war. For reasons I haven’t seen any explanation for, Sony headphones have much less of that vacuum feeling.

Anecdotally I strongly agree. I notice no such feeling at all when using my WH1000-XM3, but it's very perceivable when wearing the QC35 or QC25.

I have tinnitus, but I've had it for way longer than I've been using noise cancelling headphones or in-ear earbuds.

I noticed the same. Disabling or lowering noise cancellation in-app fixed everything.

I too feel it. Then again it might make sense that even if we don't hear it, the pressure is still there. As conservation of energy. After all active noise cancelling is destructive interference. Which has funny meaning that the energy is actually doubled. As we produce noise of equal power but opposite phase. Now does the energy end up in your ear or the headphone or outside? Or is it divided between these.

The energy is dissipated as heat in the speaker coils. Energy is removed from the air. Think of it as using brakes to slow down a car. Yes, energy is consumed to do so (that's why the rotors get hot), but the car doesn't have 'hidden' energy, just because it was both accelerated, then decelerated. It really is at rest. The energy it embodied while in motion has been converted into other forms.

Yes. This can cause sinus issues.