My brother in law had this exact same problem with his breville. Cheater cup with aftermarket handle.
The problem is because the pressure is created by forcing all the water through the one tiny hole at the bottom, so it comes out at high velocity. The normal holder is designed to contain this, collect it, and let it run down into the cup.
The aftermarket one isn't designed this way, so the high pressure steam hits the bottom of the diverter and shoots out to either side.
It doesn't matter how much coffee or how you grind it. The pressure will be the same.
The solution is to not use the cheater cup (I use it exclusively). Or get the original handle and use that.
Or maybe you could put a bit of steel wool up there? Anything to break up that stream.
Hope you get it figured out.