Not sure if this is a bug related to specifically Waterfill but if i try placing Artifial soil on gleba on land it says "Waterfill is needed to fill..."
I i place waterfill and try placing it on top of it, it says "Landfill is needed to fill Water"
And if I place Landfill it still shows the same message "Landfill is needed to fill Water"

I've seen the same/similar, think its how they have done the special soil stuff.
Its a bug (not sure with what thou) but not a bug im going to fix currently as it doesnt affect waterfill being used on land or even landfill on waterfill its just some weird interaction with the artifial soil.