This is on Minecraft 1.16.5, and I wanted to report an issue I've noticed while trying to run Cave Biomes API along with Rubidium. To actually use the API I was using both Darker Depths and the Caves and Cliffs Backport alongside it. When you have this setup and hop into any world, it seems to break the Biome blending effects and while trying to build a personal modpack with a lot more, I managed to even get it to crash. But I made sure to test these mods in isolation to confirm what I think caused the crash, and visual errors.

You can see the effects of this here: https://i.imgur.com/vtvmyzS.jpeg

I'm not sure if the extent of the issue is anything more then visual, but I also understand if this isn't something you'd care to really fix, as both adding the cave biomes along with how much Rubidium/Sodium changes to rendering, I'd totally understand if it's simply the two are incompatible. Hopefully this report is of any help though, and if it does help to post that previous crash log do let me know. I didn't post it here because it mainly reports the stacktrace of Rubidium's methods.

If the stacktrace is mostly Rubidium, you should probably report there and post the crash log there first. Rubidium devs should be aware their code is very experimental and the most likely culprit of most crashes it's involved in.