Audio tracks becoming out of sync

I've recently observed a minor bug as follows - after a recording session, recording numerous vocal tracks alongside midi, I've saved the song. I have then left the Fantom G switched on while I've gone to do something else. When I return to the keyboard and press play (note that the track was not at the beginning) the vocals are way out of synch with the midi. However as soon as I restart the track form the beginning all is well. This has happened a few times now and I wonder if itst something to do with when the screen saver kicks in?
As I said its not a big deal as there are no ill effects after restarting but it did give me a scare the first time it happened!

I’m really impressed btw with the quality and ease of recording directly onto the Fantom