The bug

Trying to interact with an entity which is 3 or more blocks away without seeing its eyes only causes the interaction to happen only client-side.

How to reproduce

1. Summon a horse
2. Make sure you are three or more blocks away from it and that you cannot see its eyes
3. Right click the horse
   → The rotation of the horse is applied to you but you did not mount the horse

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.network.NetHandlerPlayServer.processUseEntity(CPacketUseEntity) requires the player to see the eyes of the entity (method EntityLivingBase.canEntityBeSeen(Entity)). This check might make no sense because you are not always trying to interact with the head of an entity.