The bug

When a player is mounting an entity (such as a minecart, a boat, a horse, or any other rideable entity) and driving/riding away, it will reappear to players who were near the location the player mounted the vehicle, at that location, as soon as he is out of range.

To reproduce

You need a server and at least two accounts in order to reproduce this issue.

1. Build a rail track that is long enough (100 blocks should be enough with render distance 12)
2. Player A and B start off at one end of the minecart track
3. Player A mounts the minecart and rides off into the distance
   → After a while, player A will appear to teleport back to where he entered the minecart (to player B)

Note: This seems to be dependant on render distance (not sure whether server- or client-side). I tested it with a render distance of 12 chunks (both client and server) in a LAN world, in which case the player disappeared after travelling approximately 80 blocks on the minecart track. With render distance 6 (client and server), it did not work.

Additionally, in order for this to work, the player riding the entity must move their cursor or update anything else with their appearance when they're out of range.

Video

Minecraft 1.14.4 2019-08-05 00-22-02_Trim.mp4

Credit

This glitch has been discovered by Black-Hole and SteuerungC (uwmc.de devs)