I was able to download the post 21 database, view the root cause of the problem (the validation rule on Due Date). I linked to the back end database without changing anything. I got the validation rule as expected.
I then edited the back end database to remove the validation rule from Due Date, at which point I saved the table & closed the db.
I then used Linked Table Manager to Refresh the link. The validation rule successfully disappeared from the FE link, no problem.
Remove the validation rule
Regardless. That table is corrupt and likely going to get more corrupted. I could not link to the table downloaded in 21. I tried deleting the link, and relinking. Tried through the link table manager. All other tables linked no problem. The strangest thing was that I should have recieved an alert that the table did not update the link. You might have gotten lucky. Yes remove the validation rule, but that is just a band aid if you can even get there . The OP already pointed out they did that already. The likely reason it "came back" is that it never really linked to the new table with the update. So only appeared as it came back when in fact the old link was not modified.
So I warn you not put any data into this table without rebuilding. The behavior demonstrated is not normal.
1. Create a clean back end
2. import the other backend tables
3. Rebuild the tblDocIssued using a make table query. You may have to save the old primary key in this table as a number field so you can update other foreign keys.
4. Import new tblDocIssued
5. Compact and repair
6. update foreign keys to new primary key in tblDocIssued if necessary
7. Import other objects.
8. Reestablish references.
Nice clean db and you can sleep well at night.