I'm surprised nobody ever noticed this .
I'll try to fix it soon; I need to release a Blueprint fix for other issues soon, anyway.

yay!

On reexamination, I do not think this issue is Blueprint-related.
It appears that the textures for the hanging signs attached to fences are not auto-generated. Therefore, individual mod creators would need to create them. This makes individual mods responsible for adding support, not Blueprint.
See this folder containing textures for vanilla only https://github.com/MehVahdJukaar/amendments/tree/master/common/src/main/resources/assets/amendments/textures/entity/hanging_sign/minecraft

i did not realize that being able to put them on fences was not a vanilla feature. oops!