When opening discussions with many posts on iPhone (but usually on all discussions regardless of post count), most of the time you won’t get to the last unread post but much behind like 10-20-30 posts behind, so it’s not just some minor visual offset. I usually browse back and then open the discussion again and after 5-10 times it finally fixes itself until next time. That has been an issue since I started using Flarum and can be observed on any Flarum forum including here on Discuss.
I am willing to pay €100 or even €200 if anyone is able to find what the reason is and how to fix it (even if it is a problem with WebKit on iOS and needs specific workarounds for iOS).