just a workaround, if they not snap, place a square ceiling before triangle one and place a ladder. snap ceiling whitout moving mouse and you only have to move "Y" view axis with keyboard directional arrow in order to snap it right you want. it took 20 seconds instead "2 hours".
BTW i understand the problem. Also sometimes square crop doesn't snap on some foundation that already have integrated cables or pipes.

PS, have you disabled "homestead" server feature?

I have this same problem right now, but only with greenhouse triangle ceilings. The triangle plots snapped fine on wood triangle ceilings but won't snap on greenhouse at all, and the square plots snapped fine on both wood and greenhouse ceilings.

Also yeah, like Dr4g0n36 said, getting them to snap to ceilings with integrated piping was fidgety. I ended up running the piping through the walls and having the taps on the walls too, the greenhouse isn't super massive so everything got irrigated fine. I'm trying to place the triangle plots manually but it's a huge PITA and they place lower than the square plots which snapped correctly. It's super annoying.

Edit: I found a workaround that ends up having triangle plots on triangle greenhouse ceilings but it's a bit annoying. First place down a wood triangle ceiling and place the plot on it. Then try to forcibly place the greenhouse triangle ceiling over the wood one. Either the wood one will break or they'll both end up placed in the same space. If the second happens, you'll have to pick them up and try again. But if the greenhouse ceiling breaks the wood one when it's placed then the plot doesn't demolish.

nice, thanks. just another fidgety snap problem to waste my time with, im already used to that when it comes to this game, everyime i try to build a ship-looking raft i spend hours on hours trying to get stuff to snap how i want xD

Anyway, thanks guys.