Correct. I’m on 11.1.2 on both my iPhone 7 Plus and my iPad Pro and neither one does that. Just select the correct form a couple of times and it will stop.
FWIW, I had it with 'is' -> 'I.S.' ... I just used GBoard for a while. Then I randomly tried the native keyboard again and it had fixed itself. No updates, nothing. Just randomly fixed. That's after it managed to transfer itself from my 6s to my X.