Your assertion is that something is being lost, or overwritten, but as you can see from /my/activity/bookmarks multiple bookmarks in the same topic are preserved.
There are three obvious scenarios here:
1. Bookmark a post within a topic
- Can no longer bookmark the topic itself
- Bookmark in /bookmarks goes to the post
- Nothing is lost
2. Bookmark a topic, then a post within the topic
- /bookmarks icon goes to topic
- Additional per-post bookmarks are visible at /my/activity/bookmarks
- Nothing is lost, or overwritten
3. Bookmark multiple posts within a topic
- Regardless of the order they’re bookmarked in, the icon on /bookmarks goes to the lowest numbered bookmarked post within the topic (I’ve tested and confirmed this)
- All bookmarks within the topic are still visible at /my/activity/bookmarks
- Nothing is lost, or overwritten
Your criticism of /bookmarks stems from not understanding the priority for the icon. You aren’t limited to a single bookmark per-topic:
- If the topic and subsequent posts have been bookmarked, then the icon on that page goes to the topic
- If multiple replies have been bookmarked, then the lowest-numbered reply within the topic to be bookmarked becomes the destination
The bookmark icon at the bottom of the topic being swapped to ‘clear bookmarks’ isn’t entirely helpful, because if you bookmark a topic and then a post within said topic there’s no obvious UI to remove the bookmark from the topic, while leaving the post bookmark intact. For now you need to navigate to the above link and remove it from there.
I asked you if you were expecting /bookmarks to show all bookmarks within a topic as discrete rows, because in that view it only shows the topic title, unlike /my/activity/bookmarks. If /bookmarks were to behave in that way, how would a user differentiate visually between two bookmarks within the same topic without having to check the reply ID it directs to. Here’s a mockup:
How would I be able to tell from title alone which of those takes me to the topic, and which takes me to a post?
My last point related to whether it would be possible to indicate the position of all bookmarks along the topic progress bar. In particularly large topics it would be invaluable to jump between them, otherwise users would be forced to jump back to /my/activity/bookmarks periodically to find the next link.