Asking here, because the issue happened after installing Blueprint.

I added a mod (Compat O Plenty) for a custom modpack, that also includes Biomes O Plenty. Compat O Plenty requires Blueprint. Made a personal server with this, and eventually noticed no BOP biomes were showing up. Contacted the BOP devs, didn't get much info. I tried creating many test servers for what mods could be causing this BOP biome generation issue (as it would work by itself). Using the "/locatebiome" command helps determine whether a BOP biome could be found or not. After adding Compat O Plenty (and Blueprint), and running the server two times, BOP gen stopped working during the 2nd server run (without touching any files between sessions). When they are removed, on a fresh server, BOP gen works each server session.

So, I think Blueprint is causing this issue, and am wondering how is this occurring and if there's anything that can fix this.

Using Forge 40.0.24 (required due to using the latest OptiFine H7), Biomes O Plenty 16.0.0.109 (with TerraBlender 1.1.0.99), Blueprint 5.3.0.

Duplicate of #154.
This issue is fixed in Blueprint version 5.3.2.

The problem with using that Blueprint version is that it requires a higher Forge version. If to change the line in the mods TOML file to use Forge version "[40,)", I get this error: "Mixin apply failed blueprint.mixins.json:AdvancementBuilderMixin".

Yes, I discussed that with you here: #157
I warned you there are important bug fixes that are not on Blueprint version 5.3.0. Unfortunately, there isn't much I can do for you right now, but I advise you to use the latest version of Blueprint as soon as possible.

Oh, right, I had forgotten that was posted. Thanks for the reminder.
It's unfortunate OptiFine doesn't have a release at this time for any Forge version above 40.0.24, so I guess the only solution for now is to not use one of these conflicting mods.