Cracked rear hub

Just to say, the non drive side spokes on the more traditional hubs are usually less tight, slacker, due to the dishing to accomodate the width of the gear block. This has caused problems over the years and wheel building evolved into an art form from where wheel builders gained strong reputations for reliability. Obviously the fault is due to either wheel building or hub fragility, both sides will deny responsibility. Some builders, taking into account of the increased spoke angle and less tension of the non drive side may build with brass washers at the spoke bend thus taking up any play at the point due to the slacker spokes. It may be the best advice would be to seek a wheel builder of note from cycling friends or cycling hub.  

I would suggest spoke tension is too high or too low (more likely the former). I'd suggest it's a builder issue, so would suggest taking it to another bike shop for a second opinion. In the past these failures were more common, but for 2 on the same wheel from the same builder seems to point to the builder. You could take it up directly with Shimano UK. I've found that Shimano themselves take their customer reputation very seriously.

The only time I have had a flange fail was about 30 years ago when (against manufacturers advice) I had a Campag hub built up radially.  To me what you have experienced is hub failure cause by too much spoke tension.  Tiaga stuff is not expensive kit and for 2 to fail from the same builder does suggest that it is something to do with the builder, I would suggest (I hate people who do what I am about to do and diagnose a problem over the internet without seeing the physical evidence themselves) that maybe the spoke tension is too high for that particular model of hub.

Just an observation, but there doesnt look to be much flange material around the spoke holes to give long life, or extended life. A larger circumference flange or thicker

As a man of many hubs, I've never seen failures like these. Bad batch, possibly?