I can’t tell you how many estimates I do for people that have a serious problem with their grout lines falling out. There are so many different methods tile installers and builders are using in attempts to prevent tiles from cracking do to foundation shifting or for sound proofing. One of the is lining the foundation with a cork board.
This cork board is typically used in Condominiums to absorb the sounds as someone walks on the tile. In theory it’s a great idea but there seems to be problems with the floor over time. The most significant issue is when you use mortar over the cork then lay the tile, the floor now has a slight flexing issue. This is because the cork contracts and expands as pressure is applied to it by walking on the tiles. This flexing causes the floor to move slightly and when that happens, the grout gives way. The cork board is adhered to the floor by a glue type product and can also release itself over time causing more flexing to the floor.
Below are pictures showing how the cork board is placed under the tile.
Using cork board has become the standard in new condos and is effective with sound proofing, so you will probably have it installed under your floor. What I suggest is asking your flooring company to use a “Epoxy Based” grout. These grouts are extremely hard and will give you the best defense on floor flexing. There are a variety of epoxy grouts you can ask for and your installers should be familiar with the best ones for you. Just a note, epoxy grouts are a lot more expensive so your cost will be much higher than using standard grouts. Trust me it will be worth it in the long run. You don’t want to replace an entire floor if you don’t have to, it’s a mess!