Why Did My Horse/Boat Disappear When Unmounted?

When You/I Ride A Horse, The Horse Sometimes Just Disappears, But If You Ride A Boat It Also Disappears, But Its A Bit Different With The Boat, You Can Barely Move.

If You/I Want To Fix The Bug, I Need To Quit My World And Rejoin The World, But It Keeps Happening Even When You Rejoin.