Relates to MC-140210.
The issue
If a resource pack with custom background is used, the panorama in the background would be expected to show a scene utilizing said textures. This is not the case.
How to reproduce
Download and install the pack made by me.
(Link: https://drive.google.com/open?id=1Ds4S-nLJwLT92Jo782-dFlpjIumwaeK5)
The menu panorama is a pre-made file by Mojang, and can be edited in a Texture Pack. It is not generated by the game, and does not update with packs.