The important thing here is the Webkit engine. Unfortunately Apple don’t ‘backport’ Safari features/fixes to old iOS versions. So Safari on iOS 15 is 1-2 years out-of-date 
But to be clear: our browser support policy currently includes iOS 15, so we’re very happy to investigate and try to fix any iOS15-related Discourse issues. We just mentioned it here because it might be the reason that @Ed_S is the only one reporting this issue here on Meta.