On 1.82+ it does nothing.
If your image has alpha, and you simply convert it to 24bits it will look cutout.
In the past, it was possible to workaround it, by simply getting rid of Alpha Channel:
Converting it into full white 255, and then flattening it with 24 bits. Receiving a nice full image.
Now it's broken.
-
On 1.80:
Change Color Depth: 32 bits Alpha 255
Resize: 3k
ICC Convert
Canvas Resize: 3k x 3k , Alpha 255, Keep Ratio
Change Color Depth: 24 bits
Produces:

On 1.85:
Same settings produce:
