Discovered this while messing with abbie5's Packages PR adding EMI support. Link goes to a specific commit exhibiting the problem.

Packages has a tag called packages:things_you_need_for_package_crafting that currently has the following contents:

{
	"replace": false,
	"values": [
		"minecraft:copper_ingot"
	]
}

It's intended to be datapackable but by default this happens to be exactly the same as the c:copper_ingots tag, and in fact, EmiTags.betterTag discards my tag in favor of the c one. This is all well and good except it causes EmiTags.TAG_VALUES to not contain an entry for it, and creating an EmiIngredient for packages:things_you_need_for_package_crafting returns an empty ingredient, breaking the emirecipe we are adding:

I should probably not make use of tags like this (should make a real recipe type and use an ingredient instead!), but it's worrisome that having two tags that just happen to have the same contents can cause one to break the other.

Yep, ran into this yesterday as well. I plan to get a resolution out today that expands the tag ignoring EMI's caches.

This has been fixed in 1.0.10