the cross-compatibility I'm looking to solve here is with Create, and over there I see this issue where someone was trying to use modded signs in their contraptions and failing like I am right now. My understanding/best guess is that Create's grabbing all signs from a tag and (figuratively) shoving them in a "preserve the data when moving" box, but it doesn't know to grab Cork Hanging Sign (and some Immersive Engineering stuff that I'm about to go work on) without the tags on it.

Here's a comparison in F3 of a vanilla sign (with all the mods I'm running) and yours -- maybe one or all of the following?

#minecraft:all_signs
#minecraft:wall_hanging_signs
#minecraft:all_hanging_signs

Fixed in V1.5.1