Long story short, OTG (OpenTerrainGenerator) generated a world using a preset, called BBOP (Biome Bundle O' Plenty), and having RecurrentComplex with the "Whether the nature (e.g. trees, mushrooms) added by the mod should be actively generating" config option enabled will create patches of snow randomly in Jungle biomes. This can also affect any body of water, which will have ice if it's inside of the patch area. Disabling this config option does not generate the snow and prevents the issue, but makes the mod itself useless.

Running latest version of the mod, on 1.12.2

lmao that's a weird bug to have

Cascading chunk gen will definitely not cause snow in other biomes, unless the OTG mod authors have some kind of generation context going (which I really hope they don't, because it's not good practice).

Sorry, but I think it's on their side to fix. Recurrent Complex doesn't meddle with biomes at all. It just generates structures.

From OTG dev