To reproduce the bug, place a command block with command "execute positioned as @p run tp @p ~ ~ ~", and activate it with a redstone clock. This will keep teleporting the nearest player to the same location they're already in. The teleports are therefore expected to do nothing.

However, when being teleported by this command block, if the player turns around, the player's hand item will jerk to the left or right depending on the turning direction about once every full turn.

Also, the player's camera is stuck in place for what appears to be one client tick every time they are teleported. This camera freeze only affects the position of the camera, not the direction. As I show in the video below, there are no framerate drops and I can turn my view direction smoothly (even though the hand item jerk issue occurs). However, whenever I change my camera position, for example by walking or jumping, the camera freezes become apparent and it appears that the game is only updating 20 times a second.

Both of the above issues appear for any teleport but are easiest to see with a teleport that does not actually change the player position, like the command block command I suggested above.

Both issues, but especially the freezing that is new in 1.14, drastically impacts player experience on my public modded server, which features a first person shooter gameplay. To simulate the recoil of guns, my server sends teleport packets 60 times a second that do not move the player but rotate them. However, these constant teleports cause the aforementioned issues to occur in the same way as the singleplayer command block example. Therefore, players in 1.14 effectively stutter down to 20FPS (even though the FPS does not actually drop) when using a weapon that uses this teleportation-based recoil system.

Besides my public server, mapmakers could also use a similar technique with the TP command to rotate players without moving them, so the bug has implications in vanilla Minecraft as well.

This is a video demonstrating the bug. This video details how the bug behaves and how you can reproduce it in vanilla. It also contains footage from my public shooter server to help illustrate the issue.