If you in spectator mode and in non-entity-ticking chunk and the "spectatorsGenerateChunks" game rule is false, then save and quit and rejoin, your head and hitbox suddenly visible. This effect will go away when you leave the chunk, flying higher or flying into a solid block.

STR:
Step 1: Type "/gamerule spectatorsGenerateChunks false".
Step 2: Switch to spectator mode, then go to the spawn chunk, then go to the "unload" chunk.
Step 3: Press F3+B (optional) then save and quit then rejoin the world.
Step 4: Press "E" or press "F5"

Observe: Your head and your hitbox will be visible.
Expected: You will be normal like when you in normal spectator mode.