Minimum mods to reproduce:
supplementaries-1.16.5-0.12.2
betterendforge-1.16.4-1.6.0

Forge: 36.1.16

With these two mods installed, I am no longer able to copy banners. If I remove either mod, the problem resolves itself. 

EnigmaticaModpacks/Enigmatica6#2279
https://www.curseforge.com/minecraft/mc-mods/supplementaries/issues/84

Fixed on supplementaries end.