Never heard of unit that drops them after each trip unless they are being cleared by someone, maybe at another operator position on the boat.
I do recommend the AUTOMATIC track deletion so that the machine doesn't fill up with tracks and then stops tracking at strangest or inconvenient time. I also recommend that tracks are set to drop by "Distance" and not "time" because time will drop and waste points under the boat just sitting and it will eat up your tracks. Maybe that is what you have going ON. If the machine is set for time, it just uses up all your track data in a big pile under boat and yesterdays tracks are gone.