If Player A assign a block, Player B not get the updated texture.
Only if some player update the world near to the block (same chunk) (place a block, break a block, break a plant, etc...) Player B can see the new texture.

Also if the Player B change Render Distance (update Rendering) Player B can see the new texure.

Tested with the latest minecraft forge 1.12.2 version (forge-1.12.2-14.23.5.2847)

Mods installed:

blockcraftery-1.12.2-1.3.1.jar
mysticallib-1.12.2-1.3.7.jar

is this pullrequest: #43 in the latest build included?

Same here