Landfill on oil oceans (both shallow and deep) no longer gives the "foundation needed" message and the landfill is consumed. It doesn't appear to be placed though and can be infinitely dropped on the same tile without the tile being "filled."

Update: it's actually a mod conflict. Looks like this doesn't play nice with the "NiceFill" mod (https://mods.factorio.com/mod/nicefill-scriptfix)

If I had to hazard a guess, it might be because of the oil tiles being walkable, NiceFill assumes that it is okay to replace the landfill tile with them (?), I'll take an actual look at it later though.