There is a glitch that occurs in spectator mode that doesn't update the players visuals; when the player is spectating an entity and the entity is teleported a minimum of about 100 blocks through a command block or player commands. No matter how much the entity's position is changed the player doesn't appear to update anymore even if the entity does come back to the initial position. The last thing that I've noted is that when the player exits spectating the entity, the player's position does update to the new position of the entity but only when you stop spectating the entity.

It should be noted that spectatorsGenerateChunks is true, the chunks the entity was teleported to were loaded (if they are not loaded the glitch still occurs), if the entity is teleported back, the player's position still does not update (the entity appears to just pop in from nowhere)

Commands:

/summon llama ~ ~1 ~ {NoAI:1}
/tp @e[type=llama] ~ ~ ~100

Other notes:
I have tested this with a llama and a sheep

This has been a problem in 1.13 as well

Found no information in the logs files

Has been an issue on different computers

Summary:

1. Player is spectating an entity
2. Entity is teleported a minimum of about 100 block through command blocks or  player commands.
3. Entity does teleport to the new location
4. Player's visuals do not move to the new position of the entity
5. When the player stop spectating the entity, the player ends up at the entity's new position