A major differentiator since Apple’s earliest days has been interface design. Apple has long prided itself not only on creating more beautiful interfaces, but also much more usable ones. The aim has always been to make things more obvious, and also more efficient. So what happened in Mail for iOS 13?
There is a whole lot of weirdness in iOS13 and this is one of the strangest decisions in UI I have seen for a long time. When people that are reasonably proficient in using technology are convinced there is no way to forward an email you have a problem.
The button its self is not an issue, but the icon used is a lesson in hoe not to design an interface. It clearly implies reply and no further indication that another level of interaction exist beyond it.