I have a similar problem. Brown dust drops from my ceiling in June and July
through the tongue and groves.

I had two bug people into look at it. One said it was powderpost beetle and
recommended a very extensive treatment. The second said it didn't even
resemble ppb. I forget what he said it was (it was 5 years ago) but he said
insects were eating rotten wood in my roof from when the roof leaked years
ago. The rotten wood has no strength, so its being eaten didn't matter, and
the bugs wouldn't eat sound wood. He recommended I ignore it, as treatment
wouldn't last, and wasn't necessary.

I hope he was right; the roof hasn't collapsed yet.