IIRC, you made several posts in the past in regards to accessing and modifying the database through PHPMyAdmin and not directly through the forum; it's entirely possible that management of the data through anything other that UBB.threads (as each linked portion of data is managed at once via the built in routines) could be causing your issues (as 7.5.9 was simply minor fixes, and the last time the scripts where actually added to where 7.5.7 in 2012) as I haven't witnessed this behavior at all before, nor should the original post have different permissions than the other posts (edit post time perm is the only one that should count in this case).
Previous posts:
How to Automate Pruning TopicsOrphaned PostHow to Merge Two DatabasesThe only other things I could recommend is having you check other forums for similar behavior, or check if any of their other groups have permissions which would override the target groups settings (possibly a -1 somewhere in the permission settings).
If you have modified data in the database directly (outside of UBB.threads) as the above posts suggest, the only real thing I can recommend is to compare any "messed up" table data (which could really be anywhere; forum permissions, forum data, user permissions, post data, etc.) to table data that you're 100% certain has only been managed through the built in options in UBB.threads.
You could try advanced support through the support desk here, but at this point there aren't many things we'd be able to check with user to user support.
