A block display of any hanging sign type has no texture and renders over everything else.
This can be reproduced in the latest release version by running the summon command.
/summon minecraft:block_display ~ ~ ~ {block_state:{Name:"minecraft:oak_hanging_sign"}}
I’ve also included a screenshot from a previous version where they render as expected.
Also appears to affect the rendering of hanging signs held by Endermen or on the heads of Copper Golems.