Mahogany trees don't check for the block they're spawning on, so they just spawn on water blocks as long as it meets the requirements

Here's a screenshot by the way, this is in Biomes O Plenty's rainforest biome (which already generates Mahogany trees):

Same here, but I am not using any terrain mod.

completely fixed in the new update :) I just didnt say anything in here as I was implementing it still