Replicated with only JEI and Abyssalcraft.

Brewing any modded potion with glowstone dust results in Dread Plague II rather than the correct potion, for example, when brewing Coralium Plague I with glowstone dust.

That seems to be the case with any potion that doesn't have a strong variant (eg. one that's made with glowstone). This could have something to do with Forge's brewing recipe registry, but it could also be that the way I use that recipe registry breaks it (gonna switch over to vanilla's system and see if that solves it).

Switching over to vanilla's system seems to have solved it. This could be a bug with Forge's handling, or a derp on my end using their brewing recipe registry.