I've noticed twice, within a few days, that I began making highlights soon after the draft version appeared as available in the android app, and after a couple of highlights, suddenly most of these (4 or 5 maybe) were gone, while 1 or 2 remained. After closing the draft (out of suspicion) and doing the missing highlights again, the problem did not reoccur, fortunately.

I suspect some post-processing could be the cause, but that's just guesswork. I have never seen this phenomenon before, it seems a very recent issue.

The latest case I noticed was with a saved UnHerd article: