Not sure if its a bug, but thats somewhat stupid for me.
When you use charged attack of Polaris and you have wall behind you or there is no space behind you for knockback charged bullet explodes just after you launch it.
IMO its a bug becouse its obvious that bullet should not act like that just becouse there is something behind you.
Part of the projectile 'spawns behind' you, causing it to explode immediately due to the wall. The occurrence also exists with the Avenger/Faust series and likely the Winmillion.