BSD will unload the mist ball (from either the left or right mist ball pocket) upon booting if there is a ball locked in the castle lock lane. If there is not a ball in the castle lock lane then your BSD should not unload the mist ball when booting up your game.
There is a software setting for Feature Adjustments (A2.44) that will unload all locked balls at the end of each game which also includes unloading and reloading the mist ball. I tried this method and it is sort of annoying because if you have a ball in the castle lock lane at the end of a game BSD will unload the locked ball from the castle lock lane and also unload the mist ball and then it reloads the mist ball again into the mist ball pocket. If you go into the 'Adjustments Menu' to review or adjust settings BSD will also unload the mist ball and then reload the mist ball after existing the 'Adjustments Menu' even if a ball is not locked in the castle lock lane.
Bottom line, if you have a ball locked in the castle lock lane when you boot up your BSD it should always unload both the locked ball in the castle lock lane and the mist ball and then BSD will reload the mist ball into the mist ball pocket. If you have the 'Game Over Unlock' (A2.44) feature setting set to 'No' and you do not have any balls locked in the castle lock lane upon bootup and your BSD still unloads the mist ball then as Pinsider Billc479 stated you likely have an issue with your long range opto board or with your opto transmitter.
A few months ago I had issues with the mist ball on my BSD and the thread below details the journey and how the issue was resolved.
https://pinside.com/pinball/forum/topic/bsd-won-t-load-mist-ball-at-start-of-game
Gord
BSD_Ball_Lock_Adjustment.pdf