I'm having one other issue with exporting:
I'm not getting any image properties or it would seem the same proper .dds file type upon export (see screenshot 1)
And when the texture is applied in nifskope I'm getting the "blacked-out error" (screenshot 2)
The proper display of the texture is screenshot 3
(edit to clarify: the bmp into convert.io I had reduced in size before converting, also fixing the screenshots)
https://ibb.co/HF1rzT9 https://ibb.co/jVL7CnV https://ibb.co/k9PznyD