As discussed once in a discord and something I forgot to post. 5.7 introduced the following:

Added new /spectatingcamerashift command (and corresponding plus.ini "[General]SpectatingCameraShift" configuration setting, and "Spectating Camera Shift" item in the Plus dropdown menu) for configuring whether spectating another player should shift the camera based on the spectated player's current speed (true, default, as has been the case since 5.7) or center the player exactly (false, as in 5.6 and before).

However, ever since this change the spectating camera has been a little bit shaky regardless of the setting.

Brought forward by Ahmed from the jj multiplayer community.