Biomes were given y axis support in recent updates, as preparation for 1.16 and beyond. Sadly this has now been disabled for The Nether presumably due to technical limitations, but remains in The End biomes.
Examples, using the seed -1086322053329945487, tested in spectator mode.
- x1204 z799 - flying up and down around y26 will display several examples of these biome changes, despite no variance in the blocks present.
This change currently serves no purpose, and a near identical issue for The Nether,
MC-170990, was closed as a 'Won't Fix' bug. There are two possible options here, to remove the feature from The End as well for a standardised biome experience, or re-implement the feature as part of ongoing work for 3D biomes and any Cool, Awesome, Very Exciting updates in the future. Tested on both multiplayer and singleplayer, with multiple clients.