Writing a new email on the iOS Mail app does the proper text wrapping once typing a sentence reaches the right edge of the text editor. Replying to an email may do this too, but that is not always the case.
It seems like desktop browser created long standing Gmail threads or perhaps threads with images or larger HTML widths are more likely to create this issue when trying to reply through the iPhone app.
iOS Outlook seems to handles text wrapping properly:
obs: this was tried on iOS 13.3.1.