I see what you mean, timestamps of notifications seem to bracket the post that tagged you.
Maybe your notifications are "DeLayed"?
I used to rely on email notifications.
That only told me about new postings to watched threads, not "reactions" or "tagged". I only noticed tagged if I happened to browse through a new thread.
A while back we were requested to turn off email notifications (no idea why that wasn't an IT function applied to everybody.)
Now that I use the bell icon to see new postings, I see tagging too. That doesn't explain why you sometimes don't.
As an aside, a few threads still send me emails, e.g. "Can Solar & Wind Fix Everything (e.g., Climate Change) with a battery break-through?"
For one such thread, I tried "unfollow" followed by "follow without email" but it still emails.