A player was using a blue ice boat road in a nether tunnel on my realm. Boats tend to already be quite buggy.

The player was at full speed on the ice boat road and spontaneously started lagging on the server (their frames didn't drop and their PC was running completely fine). The player could then see nothing but the void around them and jittering around.

After about 7 seconds the lagging stopped and the player appeared to be exactly where they were before they were lagging and carried on using the tunnel for about 1000 blocks. They then disconnected and the realm was closed then reopened.

Later the same player who lagged rejoined and spawned in where they were originally lagging next to 2 completely reset chunks that don't have the same generation as the realms seed.

During all of this, no severe lagging was experienced by any other players online.