Unusual Waterproofing Problem for Bathroom With Dwarf Wall Shelf Advice and information on tiling and fixing tiles to a variety of surfaces

[LoftyUK]

I've recently completed bathroom which includes a dwarf wall (tiles on cement board) with bespoke shower screen around it. The top of the dwarf wall forms a shelf inside the shower. It's great apart from if you spray water directly on the screen above the dwarf wall and then a wet patch appears on ceiling downstairs. Will the horizontal tiles on top of dwarf wall hold water? (See pic)

Any ideas how to fix? I wondered maybe make the top of wall slightly sloping or some kind of cover to stop standing water?

Thanks in advance