Peter, welcome to the forum.
I’ve exported both 8 and 16 bit LZW compressed images, and 8 and 16 bit with no compression images from Mask AI, and they open quite OK in Affinity Photo (current build), also in ACDSee Ultimate 2022, and in Luminar 4 (I don’t have Luminar 3). That was choosing the “Transparent” option.
I’ve also exported TIFF “Trimap” images, TIFF “Alpha only”, and TIFF “Composite” images from Mask AI.
The Trimap, Alpha only, and Composite images can all be opened in Edit mode in ACDSee Ultimate 2022 (it does see the Alpha only TIFF as just a white on black image, but it can be converted into a mask.
However 8 bit, 16 bit both compressed and not compressed TIFF’s of the Trimap, Composite, and Alpha only fail to open in Affinity Photo, and cause it to immediately crash.
And they don’t open in Gimp either. It just hangs and requires task manager to shut it down.
I’m can’t say I’m surprised about the Trimap. Its only real use is in Mask AI, and it does load back into that OK. That said Affinity shouldn’t crash and Gimp shouldn’t hang.
I’m also not surprised that Affinity has issues with the alpha only image, as Affinity handles alpha channels differently to Photoshop. Again though Affinity shouldn’t crash, and Gimp shouldn’t hang.
I am a little surprised that the “Composite” causes issues. Perhaps it is being saved as a TIFF with layers in a format that neither Affinity or Gimp understand, or perhaps there is actually something wrong with those particular exports Would be interesting to hear whether they can be opened in Photoshop.