These are (relatively) inexpensive and not name brand rims. I've had good luck with them in the past, but I've not seen this before.
My stress relieve process is usually to place the wheel on it's side and bounce some of my weight on the rim while spinning the wheel to do the full circumference. I'll also place the wheel in my lap, put my elbows on the rim and then grab the far side and give it a pull (hard to explain). These are 42mm wide double wall rims, so fairly stiff.
I'd say max tension wasn't ever over 130kgf while building up.
Thanks for all the replies. I'm moving on from these.