Ah, I completely forgot to code this exception, sorry!
I can see what it's doing here, for the outline of the quad, it's using the UV data of the quad correctly, but grabbing the font texture, rather than the quad texture.
Here, I've coded an update that gets the right submesh data for quads: https://pastebin.com/wWv49mgG
Unfortunately, the outline generator script effects the whole mesh at the moment, unlike the text shaders. I might look into changing this if the shaders don't get finished shortly, though. (I think I can store the data in a UV channel, future Kai reading this...)