Just spotted that the WA Server Validation was inserting the first WA Server Validation title, healthscreeningform_984, in the validation for the second WA Server Validation, healthscreeningform_476.
I've updated that manually and both validation behaviours appear to be working now.
Still a couple of issues though...
I've had to append querystring data to the $WAFV_Redirect so that the correct Bootstrap tab is loaded upon validation failure, as follows:
$WAFV_Redirect = "health-screening-form.php?tab2=1&v=1";
The problem with this is that every time validation takes place it continues to add querystrings to the URL, like this (three validation attempts):
Why isn't it just updating the querystrings, considering that they have the same name?