If you are holding vanilla items that could be the crafting recipe for the disguised modded blocks, Ex: I'm holding wood, and custom retextured stairs would come up in the crafting recipe. This can be a huge problem since mod devs will just get lazy and use random vanilla items instead of barriers or uncraftable items.

Version? And are these suggestions through some mod (like Emi, Rei, Jei) or do you mean vanilla recipe book?

I mean the vanilla recipe book and I'm using the latest version for 1.21.1. This isn't for one specific mod I notice this happens for many mods.

Version? And are these suggestions through some mod (like Emi, Rei, Jei) or do you mean vanilla recipe book?

After checking, it only shows itself as craftable, while still showing missing red ghost slots after clicking (on 1.21.4 at least). Either way it's not fixable without modifying the client. Best to wait for Mojang to improve recipes / adds data driven items