I went through the code once again and after I took a little bit of perspective it turned out I've had yet another form tag wrapping the part of the code that I was working on.

Lesson learnt, always make user forms are not nested within each other.

In case anyone else stumble upon similar issues, I'm posting some errors I encountered:

• Parameter Missing or Invalid: Items
• Double post actions in the network tab
• Missing first form tag when iterating through variants