I've had this issue on many different versions of Altium. Sometimes when interactive routing from the end of an existing track that is not completed the silly router will flip over to another layer and start routing on another layer
I find I have to delete tracks back to the nearest pad and restart the route again. The other thing it does time and time again but not always is to start a route with the track keepout option enabled
Wonder if anyone else has experienced this and perhaps there is a simple fix for it but ca be very annoying after a while ?
cheers