I believe there is another issue with the manage relational table behavior. As far as I can see, the fix works if there is only one manage relational table behavior on the page. I have an Insert page with two manage relational table behaviors. Only the last one added works. For the first one, nothing is written to the relational table. The second one works as it should.
I'm attaching my page. It's a Product Insert page with size (størrelser) and color (farger) options. In this case, only the color (farge) is inserted into the relational table.
I'm also attaching the wa_appbuilder_php.php page that I'm using.
Live URL here: products_insert.php