Even though it worked perfectly for other posts. It also work fine on my offline WP.
What you might want to know are:
1. My blog is on WP MU 2.5.1
2. Simple Tags Version 1.5.7
3. The tags on those 2 posts are not displayed on "Tags" and "Mass Edit Tags" pages, but not on "Manage Tags" page.
4. These posts can still show related posts on themselves, but other posts won't display them.
5. I already tried to delete and recreate those 2 posts, not getting better.
I wish someone can give some sound solution while I'm trying out. Thanks very much.