This issue has been in effect for years. I was hoping it would have been reported and investigated before now, but it wasn't.

This issue affects ALL resources from global resource packs. The background cubemap is the easiest to test.

Steps to reproduce:

1. Download a third-party resource pack from a community website e.g. MCPEDL. Try one that changes the loading screen for faster results.
2. Install the resource pack.
3. Add the resource pack to the Global Resource Packs.
4. Start a world, new or pre-existing.
5. While the world is loading/generating, the background cubemap will change from the resource pack's cuebmap to the vanilla cubemap.