Desync occurs when a mob starts riding an entity that the player is riding, and the player switches to spectator mode in the same or previous tick

Steps to reproduce the issue:

1. Enable the spectatorsGenerateChunks gamerule:
   /gamerule spectatorsGenerateChunks true

2. Ride a mob (example: horse) and run the following commands with chain command blocks in order:
   /summon chicken ~1 ~ ~
   /gamemode spectator @p
   /ride @n[type=minecraft:chicken] mount @n[type=horse]

Expected result:

Chicken starts riding the horse.

Actual result:

The chicken visually stays where it is, but cannot be hit. Technically, it is riding and shows up in the data get command. (See video clip)
When killed, loot is dropped from the correct location, so it only appears to desync visually.