spa wall leaks water when the pool is running,

Welcome to TFP.

Do you have a spa spillover?

Show us pics of your spa walls.
 
If you examine the grout lines carefully I am sure you will find cracks in the grout where water is entering and getting behind the tiles and entering the gunite. The water then looks for spots to drain out of the gunite.

The water behind the tiles erodes the thinset and lossens tiles which is why you lose tiles.

While the small format tiles look cool they have many more grout lines to cause problems.

I have larger format tiles and give a good look at my grout lines every season looking for developing cracks that I regrout. It has become part of my annual maintenance to keep my 20 year old tiles intact.
 
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