I have the same problem of sync wave/virtuals synth. Specially when the beat is sliced on each bars. I have the same problem with the 2 computers, 2 differents soundcards. So I stop the sequence, then restart and it's ok for a while.
It's remind me the old Emagic on PC when the syncro was a mess. Come on Bandlab, fix this please. I think seriously to change for cubase or an other more stable!