Our's also does that, despite cleaning etc.
To get round it, you have to wait until after the soap powder cycle, then add fabric softener into the powder section in the drawer during the rinse phase. Works in our washing machine, fabric softener gets to clothes effectively, though it is annoying as you can miss putting the fabric softener in.
Probably more to do with the pressure of the water that washes the stuff out of the drawer into the machine, rather than stuff physically being able to get out of the drawer. Thinking about it, my Mum uses tablets that go in the main drum, maybe this is for the same reason..?
Not sure if this helps or not!