I'm trying to setup a SevTech 3.1.2 server, which includes JustEnoughIDs-1.0.2-26.jar and jeidsi-1.2.1.29.jar.

When I add spongeforge-1.12.2-2838-7.1.7-RC3900.jar, all the chunks it generates have the plains biome. If the chunk was previously generated with Forge, the blocks still look correct when SpongeForge is added. I forgot to check if the biome id is correct in the minimap.

The effect of the bad biomes is most visible in the TwilightForest:

The biomes look correct with spongeforge-1.12.2-2838-7.1.7-RC3785.jar. I'll try doing a binary search to narrow down which build it broke in.

Note that to use a new version of SpongeForge, one has to remove the streams and farseek mods from the modpack.

I'm filing the bug here because someone on the SevTech discord said JEID doesn't work with SpongeForge.

The last build it works correctly on is spongeforge-1.12.2-2838-7.1.7-RC3874.jar. The next build has the plains biome problem.

This is the only change in build RC3875:
SpongePowered/SpongeForge@fbe7f76

I see version JustEnoughIDs 1.0.3-44 lists '[~] Fixes to help support SpongeForge (thanks Zidane!)' in the change log.

Should I retest?

I just tested in my own instance with a few different biome mods, Twilight Forest, and the latest SpongeForge (spongeforge-1.12.2-2838-7.1.7-RC3903) and there doesn't seem to be an issue with JEID 1.0.3