Hi! I can't reproduce this issue. I have tried with your modpack, as well as some of my own saves, and can make diamonds.
If it helps: I only enable diamonds in data.raw["furnace"] and data.raw.["assembling-machine"] entities with .energy_source == "electric", and a .crafting_categories entry that matches "smelting".
I do this in data-final-fixes so if anything changes after this mod in data-final-fixes, I won't catch it.
Not sure if that helps, otherwise I might need an actual save file to investigate further.