@Martijn > Welcome!
The sync, if enabled, will take the tracks and match the tempo to the deck set as MASTER. So if your master track (let's say Deck B) was at 130BPM and your other track (on Deck A) is at 122BPM, and you press SYNC, Deck A will change the pitch to roughly +6.55% to match 130BPM.
When the first song ends, the 122BPM track (now at 130) becomes the master, it stays at 130BPM. A new track being loaded on Deck B, whatever the tempo, will have to match 130. If the track is un-analysed and doesn't have a tempo, it can't sync.
If you're able to repeat the process, could you either shoot a short video or take screenshots so we can better understand what might be happening?