Hello Jens,
As you said, the extractor checker (RSA3) is correctly displaying the data as required . So it is now clear that Issue is something around on the BW Side . I assume here you have done the required in RSA6 T code ( i.e uncheked the HIDE Field or that particular field. )
Suggest you to trigger your Infopack (with filter, if possible) and try to populate the data. This may give you some hint what is going on while populating the data.
Hope this helps !
Regards
YN