Exploding tracks/stems syncing issue

So the mix engineer for an artist I’m working with right now hit me up today saying the stems I sent him don’t sync up with the 2 track mix I send to the artist that he records his vocals to. So when the engineer gets the beat stems from me and the vocal stems from the artist they don’t sync up

I only use the mpc. I don’t have a daw or anything else. I bounce the 2 track from the mpc to send to artist for him to record to and explode the tracks and send to the engineer. They should sync up correct?

Is there an issue with the mpc possibly having a slight delay on exploding vs exporting?

I’m convinced there’s not and the artist is messing with the 2 track during his recording. Just wondering if any of you have ran into this issue with the mpc

What are they using to track vocals with it?