UPDATE:
The ‘remove_filter’ IS working. Something else is wrong. All I can say for sure is that if I remove the lines that adds and removes the filter ‘woocommerce_order_get_items’ then the end customer receipt(s) looks fine, but that of course creates a bug in the vendors email receipt where commisions should be shown. I need to workaround this until I have better time to debug this crazy thing. So I will disable the vendor e-mail and vendors will then just have to look their orders instead. Any information that can clear out my issue is still very much appreciated.