isset is a ternary expression, in other words, a short hand if statement
the page is cehcking to see if validation has failed and the invalid URL variable is present:
if it is, it is using the validated enteries binding.
The problem is that the false part of the expression is a blank string.
change it so the false part is your recordset:
<?php echo((isset($_GET["invalid"])?ValidatedField("userupdate","my_Field_name"):$row_SecurityAssisttbltraders['my_Field_name'])); ?>