Is this not a tool proficiency? Herbalism kits are equipment and they refer to a proficiency for them, but the proficiency isn't included in the list of tool proficiencies.

Usually if a proficiency isn't included - you already have it.

That's great, if you make a new character, and don't add anything to it, a completely fresh character. And then you add the new UA race custom lineage that has nothing that you don't select, then you grab a background that gives a tool proficiency, you can see that it doesn't come up in the list. It's easiest to tell by selecting custom background, so you can be sure that its not just granting it to you in the background. It doesn't appear in the list. If you grab the skilled feat in the race section after that, you can see that it is in the list of ALL proficiencies, but almost nothing that grants a proficiency does that. Also, the list of your proficiencies shows up in the character sheet, so NONE of my characters have it, but also none of them can select it without getting the skilled feat.

I just tried that. New character - Custom Lineage with no options chosen - Custom Background 2 skills 2 tools. Herbalism Kit shows up in the list.

If that's not working for you - link a character that you're having the problem with. Mods will be able to take a closer look at the problem.

Huh. I just spent a while trying to figure out why it wouldn't work, maybe I'm just going crazy, because it works for me now.