Same things happening to me and quite a few others on this subreddit, from about half way through September pages just aren’t being indexed. I’ve had 3 or 4 pages actually index out of about 130 now.
Are you using RankMath at all? I’ve had a few people mention they are, we are too.
I submitted a ticket to them but they responded with a definition of what “Discovered – not indexed” means (I already know, I was asking WHY is was happening) and then they said it was because two of the 130 pages are too similar so it was probably because of duplicate content, which doesn’t explain the other 128 but that’s RankMath support for you.