Thomson regard face plates as consumables that should be replaced eventually and I agree with that and like the way they don't charge the earth for replacements. It's a shame other makes don't but then Thomson stems are a bit unique in that other designs come and go but thomsons stay popular and hold their price. I'm not convinced on CNC over forging for stems but cracks are rare and i'm sure they know what they are doing. They only charge something like £6.99 for a new faceplate anyway which reflects there attitude that they should be replaced.
Could have been done up to tight or unevenly but another reason could be your bar diameter is slightly out of tolerance. And yes the dirtiness of the crack face on the right of the picture suggests it's been cracked a while.
I'd just buy a new faceplate and be thankful it didn't completely fail on a downhill bit leaving you flying forward holding the bars.