First time i've heard of it. Alpha mask is a relatively new addition to the bmp format header and nexxt does not support it.
If i had to guess, that other tool you're using is interpreting the bitmap and superimposing the mask in absence of one, but it's just my guess.
Sorry for the delay, I have let several days pass to investigate if something changed when exporting and I found something interesting, now the image saves correctly, but if I compare it has a few different sectors: Files