@the tricksterI agree, it's all speculation and I think I made...

@the trickster

I agree, it's all speculation and I think I made that fairly clear in my previous post. It's up to the engineers to figure it out.
But for me it's worth clarifying a couple of points, just for the record.

I reviewed the video in more detail.

Justin put both gen 4 buds back in the charging case with no problems. He then brought the charging case near the gen 3 device in his other hand and the feedback started immediately. The feedback immediately stopped when he removed one of the gen 4 buds from the case. So the Gen 4 buds were definitely squealing while in the case where there's no IR signal to power them up.
Did Justin accidentally bring some early prototypes?

Yes, as he put the buds in his ears he mentioned that they were synced to his phone, but I don't see how that matters.

To say "the noise is a classic audio feedback situation" is a rather vague statement. You should at least indicate the complete pathway for the feedback loop and indicate what you might see at each node.
For starters the audio volume from the buds appearing at Justin's lapel mic would have been close to negligible compared with other sounds in the studio such as voices. Inside the ear canal is different because the small confined space allows a small amount of energy from the buds to generate a significant sound pressure at the eardrum.

As I described previously, there is still the possibility that the partly powered up gen 3 was involved which was very close when the problem occurred. But the gen 4 devices in their charging case should have been fully disabled and unable to respond.ck