The articles aren't 100% identical, but it's unquestionably the same story, and when HN has had a major discussion about a story recently, follow-ups count as dupes unless they contain significant new information [1]. Commenters in the original thread were well aware that the pricing has gone up. Indeed, that was the only significant thing about the story, so I don't think there is significant new information today. One way we check that is to look whether the comments are saying the same sorts of things as previously, or whether they are focusing on something new. In this case the discussion seemed much the same.
Front page slots are the scarcest resource HN has [2], and optimizing for curiosity [3] requires minimizing repetition. Follow-up posts on controversial topics often get a lot of upvotes, but because they're mostly repetitive, it's an important function of moderation to downweight them. I'm sorry your article got caught in that mechanism. I know it sucks to get downweighted off the front page.
I completely agree with you that their announcement was unclear, and in fact was having a conversation by email about this with some users yesterday. You can see one artifact of that conversation here: https://news.ycombinator.com/item?id=22003612. HN's title rule calls for keeping original titles except when they're misleading or linkabit. Bland corporate press releases that are designed to bury the lede are misleading [4], so we tend to change them. If you want to suggest an accurate title, we can still change it. The only reason I reverted "Postman announces 50% price hike starting Feb 2020" is that at the time, I had no easy way to be sure that that was true.
I hope this helps explain things a bit, and sorry for the disappointment.
Edit:
I haven't had a chance to look into the details of what happened with those old posts you've linked to. It's possible that that case differed significantly from this one. But it's also possible that we just missed the duplication. We don't come close to seeing everything that gets posted to HN, so we miss a lot. For that reason, it's impossible for moderation to be consistent [5]. Our principles are consistent, but the practice isn't. If you or anyone see a post that ought to have been moderated but hasn't been, the most likely explanation is that we didn't see it [6]. In that case you can help out a lot by bringing it to our attention at hn@ycombinator.com.
1. https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
2. https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
3. https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
4. https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
5. https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
6. https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...