This is DJing - this is what DJs do.
The chances are one of the tracks is 128.001 bpm and the other track is 128.004 as an example… so given a few bars, they’ll drift.
Try mixing the same track into itself, that way, you’ll be mixing 128.003 into a track that’s 128.003 as an example again. If that same track won’t hold in the beat then there’s other possibilities like a platter thinking it’s being pressed briefly when it isn’t etc - but it’s much more likely to be the different BPMs