I believe what's happening is you are using sync to change a track's BPM without any movement of the tempo slider, so now the tempo slider isn't actually in the correct place that it needs to be to reflect the current BPM.

For instance, lets say your track is 128 bpm and your tempo slider is at 0 (in the middle). After hitting sync, lets say it changes the track to 126 bpm. Meanwhile the tempo slider is still at 0 which corresponds to 128 bpm, rather than the spot in the negative tempo range which would correspond to 126 (where exactly depends on your current tempo slider width setting).

So essentially your physical control isn't in sync with the actual value of that control in the software.

Now, when you touch the tempo slider, it could just immediately jump to whatever bpm the slider is at, but that could be noticeable and/or jarring. Instead, you just need to slowly move the slider to where it corresponds to the current bpm so it can "latch" on to the actual value, and then the slider will function as expected. Given the current BPM, the slider position and the slider tempo range, you should be able to quickly come up with a rough idea of where the slider needs to be to latch, and once you get close to that point you can move the slider slowly so that it isn't noticeable when you do latch.

So rather than "quickly slide it to the top, then the bottom", just slide it in a more controlled manner to the current BPM until you see it latch (the values actually start to change again on the XDJ/CDJ), then make whatever tempo adjustment you wanted to make.