When a track which is currently the MASTER ends or is stopped, while in SYNC mode, the new track becomes the MASTER, and the tempo will not change until the tempo fader on the new master track is moved, at which point it will jump suddenly to whatever tempo that fader is set to. I am currently using the Kontrol S4 MK3 with faders set to Absolute mode, not Relative mode.
I currently try to work with this by remembering to set an approximate tempo on the next track's fader before switching over, and if I forget, trying to move the fader very quickly to prevent a sudden tempo jump.
I'm suggesting an improved setting that is common on Pioneer gear, where the tempo on the new master track will not be changed until the fader is moved to the exact position that corresponds with the currently set tempo, from which point it can be adjusted smoothly without any sudden jumps.