A quick-fix to this would be to use epoxy grout around the shower area and catch those edges with it that are showing too.
Epoxy grout is water proof and not water resistant, like a cement based grout and even a flexi-grout... Although a flexi would usually have a lower water absorption rate than standard, I think an epoxy would stop water getting through the grout fully around the shower area.
If the toppfix isn't water resistant it will break down when it gets wet therefor not taking the surface off the Wedi. Just rake the grout out around the bottom few tiles and then wet what you can in the grout joints till the tile comes free.
Re-fix using a decent adhesive and then grout using a decent grout, maybe still epoxy.