When the gamerule spectatorsGenerateChunks is set to false, spectators won't be able to load any chunks that aren't currently loaded, even if they have been generated already.
When first switching into spectator mode, all loaded chunks will stay loaded. But when moving to the edge of currently loaded chunks, new chunks won't load. When moving outside to the currently loaded chunks, it shows 'Waiting for chunk' in the F3 screen. When switching back to creative mode from the same location, all other chunks load.
When taking another player's view in spectator mode, the same issue still happens.
This is especially problematic when dying in hardcore mode: when you die, the gamerule spectatorsGenerateChunks is automatically set to false (