I understand your frustration but the Adobe folks will probably tell you that this is as designed. I am not an Adobe engineer, just a fellow user and this is how I understand the preview database is implemented after being surprised myself I could not directly decrease the size of the database and researching the issue. Discarding doesn't delete the files instantaneously. It just marks them out of date and in the background the actual files are deleted over time and only if they were actually not needed. Standard previews are generated automatically for images in your database that don't have them if there is some request to see them which is triggered by just browsing through the folder in Classic. So while not automatic, just browsing through will kick off a regeneration of the preview.
Think of discarding in its literal meaning as casting aside or putting in a trash bin. That doesn't delete the items immediately but marks them as not needed. What your discard action does is akin to sticking a "get rid of this" post-it on the thing you're discarding. They are not actually gone until somebody permanently deletes them. There basically is a clean up service that goes through the building (your preview database) and takes out the trash (the previews you marked discarded). It can take a while before it gets to the room you're cleaning up in and even then it might decide that "hey they actually did use this after they put on the post-it so I am not yet getting rid of this quite yet".
This is what Google's AI says when comparing discard vs delete which is consistent with the dictionary meanings of these words:
"Discard" often means getting rid of something unwanted (like trash or a draft email) with a sense of personal choice, while "delete" usually implies permanent removal, especially in digital contexts (like deleting a file from a computer) . In software, "discard" might hide something from a view but keep it in the database, whereas "delete" permanently removes it, though the specifics depend on the application's design (e.g., some apps use discard for your own cards, delete for anyone's).
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