Expected:

Loading world resources (resources.zip placed in the saves folder) would have the same interaction each time.

Actual Result:

The first time loading a world after starting Minecraft, the resource pack bar shows up and the game is frozen with no chunks loaded until the pack is completely loaded.

Subsequent times, the world resources would load similarly, but there would be chunks loaded, and command blocks would run for one tick, instead of waiting for the pack to load first.