Interesting. I cannot seem to be able to post anything via iOS - the return key just creates a new line...
I sometimes have the same issue
What I usually do is retype the last character and immediately tap "return." This seems to happen especially when my last character is punctuation
Thanks for that tip!
You're using a custom keyboard.
as far as I can tell, it's the build-in, bog-standard English (US) qwerty keyboard of iOS. Not swiftkey etc.