Before:

- FlipCycleTileLarge_2.png (3.57 KiB) Viewed 527 times
Image > Convert to Colours -- Colours 256 -- Dithering None

- FlipCycleTileLarge_2_256.png (2.99 KiB) Viewed 527 times
Duplicated??
Edit:
Are you using XnView Classic or XnViewMP?
The XnViewMP function Image > Change color depth... converts the image transparency to white rather than black, resulting in a white rectangle as you posted above, as the source image is pure white:

- FlipCycleTileLarge_2_XnViewMP_256.png (1.08 KiB) Viewed 519 times