Loose 3/4" glass tile on row above tub. Any advice?

I had my bathroom redone about two years ago.  It was gutted to studs and redone (by a contractor).  About 1 year after the remodel a half dozen tiles came loose along the tub on an exterior wall.  The tub is cast iron (Kohler), the tile is a glass  (Hakatai) that is about 3/4" x 3/4."  The contractor came back and popped off the loose tile, ground out the thinset, re-attached the tile and re-grouted.  Those seem to still be attached.  A year later and I have about 12-15 loose tiles in the same row (just above the tub on the exterior wall).  Contractor came back and is unsure why it is happening, but agreed to fix it.  My concern is we don't know why it is doing it, so it will probably keep happening and I assume he will not keep fixing it.

I know nothing about tile, but have a theory.  The cement board only goes down to the lip of the tub.  This probably leaves a good 1/4"-1/2" gap between the cement board and the top of the tub.  These tiles are small, so the surface area contacting the cement board is probably 50% or less.  I imagine people use these small tiles all the time though, so there must be a way.  Is there some sort of material that you would fill the gap in with, before attaching the tile?  Seems like anything rigid would crack since you would be bridging different materials (cast iron tub to cement board), with different material properties.

Does this theory sound at all correct?  My other theories are the grout is bad (but why just the bottom row), the caulk is bad, there is water behind the cement board somehow.

The black area (below the cement board, behind the tile, above the tub) in this image is the void I'm worried about.  Looks okay with full size tile, but when the tiles are 3/4" it seems like it may not have enough adhesion. Just wondering what the correct fix is.  Thanks!

Make sure the tile guy uses a concrete bonding agent in the quick set (tile glue) ,It comes in a gallon sized bottle, I use it on all my tile, stone, and masonry jobs. Also make sure the grout is sealed with a silicone sealer. Use the squeeze bottle its the easiest to use. Seal it at least twice. When it gets wet the water should bead off, then you have a tight seal.

Thanks for the suggestion.  I will pass it on to him when he comes back.  I know we didn't seal the grout, but I thought it said it didn't need to be sealed (can't recall the brand right now).  I'm sure it can't hurt though.  Is it okay to seal 2 years after installation?

Did I create a adhesion problem by applying 3/4" tile?  Seems like my surface area for adhesion is less than 50% for sure.

The cement board should have been notched to fit over the tub flange. Maybe there wasn't enough (thickness) cement board to notch. If this was the case, I would have filled the gap in with quick set, thick enough so the tile would sit flush with the rest of the wall. It might have taken a couple of applications of quick set to do this but it works well. Then grout between the last tile and the tub. I also use concrete adhesive in my grout. It doesn't take much to make a great bond. Then make sure it was sealed. But two years later? make sure you get all the loose tile up. Get a small putty knife and check them, if they're just a bit loose take them out. you can soak them in water and the quick set will come loose so you can reuse them. (soak them over night). Scrape all the old quick set off the wall so the tile sits flush. Use new quick set with glue, set the tile. Wait until the next day, grout,  then three days later you seal the tile. If you have to use the tub put plastic up from the top down so the grout stays dry. I hope this works for you. Good luck!