You have to change the timestretching mode. It's the thing that I change at 5:52 Then when you do the 2nd song the 1st will stay in sync. Make sure you timestretch the 2nd as well so you can either add a 3rd or change both to be at a completly new tempo.
Make sure you set the timestretching mode to stretch after you get the correct bpm and click synced up. That's like the "save point" of the process, if you forget to do it before moving on to another track it's gonna stay at its original pitch. Try slicing both vocals and instrumental at the 1st clear kick in the track (remove the audio before the kick) then auto detect the bpm from the instrumental and tell FL to set the bpm of the project to the detected bpm. Then line up both on the 1 and see if it sounds correct. If you're doing Hyperpop acapellas often the FL bpm detection beats online sites that don't give decimal info. 90's alt can be harder due to live instruments drifting in tempo, also occasionally having more unique time signatures
