Reproducible BUG
The URL to my test pages is here:
ppmemb.php
The validation was CLIENT side.
I worked again with this page and think that the problem is how the forms are specified. You are using ".forms[0]" in the validation code in the FORM tag. But the table that was added was ABOVE that form. So when the code is re-examined upon re-opening the page the position of the form in the page hierarchy has gone from ".forms[0]" to ".forms[1]". I am guessing that your code in the extension does not take this into account?