Mortar in retaining wall and porcelain patio tiles is soft.

Concrete releases moisture from the slab's surface as it dries. But if there is too much moisture in the air—also known as high humidity—the moisture from the concrete won't evaporate quickly. And instead, too much moisture in the air will slow down the drying process of concrete. The same goes for cooler weather.

Sounds as though the wrong mortar or adhesive was used I’d get back to the builder if I was you

Firstly, is this outside patio porcelain?? The tiles have probably cracked because one possible reason could be that there isn’t a full supporting bed of mortar mix under it. If the installer has only put mortar around the edges of the tile and a few dabs in the middle then you’ll end up with unsupported cavities which could lead to tiles cracking especially if they’ve been already hammered into place and may have a weakness in them. The reason you have sandy mortar is either there wasn’t enough cement in the mix .. we use 8 or 9 shovels of sand to every 12.5kg of cement OR If the mortar isn’t wet enough i.e. not enough water added to the mix, to create the necessary chemical reaction for the mortar to harden. Or possibly a bit of both! Regards Justin

Sounds like they've got the mix wrong and the mortar hasn't hardened properly

Sounds like they have used incorrect mix or too much additives maybe and probably not used enough bedding underneath tiles probably no slurry either

Wrong ratio of sand cement

Wrong mortar ratio and by the sounds of it not a full bed which has what led to the porcelain cracking, would definitely have the builder back and have him either redo or argue for a refund.