I build a slope for the outside of a building, and rain will leak, but when I have slopes inside as well, they don't. in this image the blocks on the left that aren't leaking have 2 slope, one above the other, the ones that are leaking are just single slope blocks. I hope that makes sense, if not let me know and I'll do some screen shots of what I mean.

I dug through the code and this happens if the block doesn't have a solid top surface. A sloped top does not count as solid, so the water drips through.

Oh, I think I see, so the "inverted" block is seen as "solid" since it's flat? on top

Yes, must be flat on top. They could have checked the bottom of a block for solidness as well which would have circumvented this issue.. but they only check the top.