most likely, the data assist search behavior is set to trigger on the WADbSearch1 submit button.
In one search form this element is submit button, in the other it is an image element type.
An im age element type does not have a value attribute. Instead of passing a value, it passes x and y coordinates of where the mouse clicked it.
the code for triggering a submit button would look like this:
if(isset($_POST["WADbSearch1"])){
to trigger for the image element type would look like this:
if(isset($_POST["WADbSearch1_x"])){
you will need to edit the trigger to use either:
if(isset($_POST["WADbSearch1"]) || isset($_POST["WADbSearch1_x"])){