Cant get my slabs/stairs to be waterloggable with custom liquid 2021.3

whenever i click on a slab type or any other type that isnt a full block, the waterloggable checkbox goes dark and i cant check it anymore, i dont know why it does that and its not letting me waterlog anything with my custom liquid, maybe it does it for water but not a custom liquid like water, im not sure why if it has the same code as water.