This is a significant defect. It seems to me that the only way out is to dismantle and reinstall.
Buy expanded concrete, drill holes in the joints and try to foam. No guarantee it will work, but if the tiles are expensive, as an option to save yourself from a complete redo.
39_4 Cerezite has a repair compound (I think CX-15), very strong, does not shrink, but does not hold on vertical surfaces. Maybe you can fill the voids...