Extend Trx WZR1/2/3 with new tab and custom z-fields of table WBRK via BADI WLF_SCREEN_ENHANCE_EX - Not saving value

Question

I followed the proper SPRO-Node to implement the above mentioned badi. Like this:

It all twerks as intended:

I created the dynpro as subscreen in a function group, where i placed that field and also two function modules as accessor / mutator for the entire header struct C_KOMFLK :

BUT:

1. My hit on enter after entering a value, clears that value... maybe I need to do sth in PAI/PBO of my added subscreen ( but when reading a proper settlement doc, it gets populated during first entry )

2. I cannot save it. I also implemented the classic BADI WLF_ADDITIONAL_DATA2, but whatever proper method I jump in and implement it, I cannot pass the value of the field to get saved.

Somehow it get's lost inside the standard ...

Do I need to implement another badi ?