Happens to me from time to time, but I think it happened on 3.4 also.
I’m not sure what I’m doing exactly but the lead switches to the song I just loaded, if sync is active it messes up the live tempo.
I also have the same bug with my prime 4+. I use the lead function in conjunction of sync (sync bpm) to match the bpm for quick speed adjustments without the need of moving the pitches, and can confirm lead does not stay in the desired song and sync does not always work. I found lead works only if the track is not playing; it must be stopped, and sync bpm only works of the bpm speed is more or less than 3 bpms with respect to the playing deck. For example: if I have a track playing at 124.00 bpm and then hit lead to another at 120.00 then press sync in the playing deck, it does nothing.
I’ve been having some issues with sync & lead as well. Now to be fair, I really don’t know how to use or even change the lead deck. However randomly the deck I load a new track to causes the playing deck to sync to the newly loaded deck & sometimes this creates absolute havoc!! If anyone is willing to try explain how this is supposed to work or what Im potentially doing wrong, it would really be appreciated.