Ocean Monuments appear to be spawning in fairly arbitrary biomes; I am right now looking at an ocean monument which spawned under a Meadow/Bog area:

In my current survival world, I have ocean monuments far underneath deserts (so the water isn't visible), and in other unlikely places.

Vanilla monuments should only be spawning in Deep Ocean (with some admittedly complex rules).

This is MC 1.12.2, forge 2501, Lost Cities 1.0.1, BiomesOPlenty-1.12.2-7.0.1.2298. Plus other mods.

Source of problem found and described in #52

Same issue here.

I'm not a pro coder, but based on what Esotericist said (monument generation screws up if allowedBiomeFactors is non-empty), I poked around in the code a bit and noticed that having non-empty allowedBiomeFactors results in a LostWorldFilteredBiomeProvider being used. This provider's "areBiomesViable" method always returns true, and so (AFAICT) the ocean monument's biome check will also always be true, thereby spawning monuments regardless of biome. (Sorry if I'm off base here, but I think this is the issue.)

I am unsure if this mod is still supported, but I hope it is, 'cause it's cool stuff.

Probably fixed by @esotericist