This does sound very much like a duplicate of MC-158414, but it is slightly different (they don't teleport back, they stay frozen) and also affects the newest release version.

THE ISSUE:

Whenever another player on a multiplayer server is riding a boat, a minecart or a horse (presumably other entities as well, tested these three) and ride away from you, their player stays frozen at the point in which they left your range. They still turn their body and head toward where the actual player is facing, but otherwise they are stuck (at least that's what your client thinks). You cannot damage them, but they can block your path. (which is very annoying when you're racing on a boat racetrack)

REPLICATION STEPS:

for a more controlled test environment, build an optional straight test track.

Have a friend hop onto a rideable entity and ride away from you. after a few seconds, the entity they're riding should disappear and they look like they are just standing there. you can now approach their ghost and even touch them, as long as you don't get close enough to them that they return into your entity render range.

UPDATE:

Now I have recorded this behaviour on video. The vid itself is unfortunately too long and big for the bug tracker's 10MB limit, but here it is on youtube: https://www.youtube.com/watch?v=T6icK1QmDxQ

NOTE:

Full disclosure, this was only tested on a Paper/Spigot server. it is possible that this issue is caused by the plugins. I cannot test it otherwise.

Attached are two screenshots of players being stuck in place. One looks like they're inside a block, because they were in a boat, while the other is floating because they were on a horse.