It would appear as though your tropical island biome placement likes to put itself in cold climate biomes, 1 instances I observed are generally a non issue because one was it combined with another mod's island biome (which just happens, although said biome is more temperate and next to ice bergs), but the main instance that I observed would be the islands spawning in the middle of deep cold oceans, not sure if this is a bug or not but I imagine they would be spawning in warmer waters, also it seems as though there might be some slight issues with terrain gen mods like Tectonic that make the oceans deeper, in the same cold oceans it was putting the tropical island biome on the sea floor, nothing that generates on the island were generating on the sea floor so this isn't a big issue, but I thought I'd let you know

But the main thing is the tropical oceans themselves being in cold climates