Random occurence, location does not matter to this glitch. This also randomly fixes itself. happened 2 mins before this screenshot. I was then able to replace.
cheat TP D6 433817 530925 198
Same as Martyn I've also had this random ceilings placed on brig and galleon "unable to place due to no foundation support" even though it is in the middle of a deck, or above a solid deck. All other ceilings around the specific location will place fine.
on EU PvE