?Hi STAGE_TEN,
Thank you for taking the time to reproduce this issue on a test app and writing up such detailed instructions. I was able to reproduce the issue on my end, and I think you might be onto something, it does appear to be related to Safari ITP. We have a possible fix from a while back in anticipation for this Safari update within the @shopify/koa-shopify-auth utils, but we’ll have to do some tests to verify that it does indeed work. For the time being, there’s no workaround I can think of. Please bear with me while I dig into this issue a bit more, and I’ll share the results ASAP.