I created a minimal instance to reproduce this consisting only of:

FerriteCore 2.0.2-Forge
Immersive Engineering 1.16.5-4.2.1-131
Minecraft Forge 36.0.43 (also tested with 36.0.35)

Without FerriteCore, the Feedthrough Insulator multiblock forms just fine, no problems. With it included it won't form at all.