H13 die casting die cracking at ejector pin holes after 8k shots — need advice

We run a 630T cold chamber machine, H13 dies for an aluminium bracket, EN standard, CE marked setup. Die went into production 3 months ago. At around 8,000 shots we found heat-check cracks radiating from the ejector pin holes on the moving half — 4 holes affected, longest crack about 12mm. We've been running die temp 180-200°C, die spray every cycle, and we did stress temper twice before first shot. Pouring temp is 680°C, cycle 65s. We already tried polishing out the cracks and re-tempering, but they came back within 1,500 shots. This is now holding up a repeat order from a German customer and I can't ship with this risk. Anyone seen this on H13 with ejector hole geometry? Is it the hole edge radius, the tempering, or something in our spray pattern? Appreciate real numbers, not textbook answers.