If you pull down on the home screen to invoke a Siri search and type just an s the kb will quicky crash and return. I can replicate this on iOS 18 15&16PM.
For me there was an update today that fixed it - 18.0.1I posted here about the same thing about a week ago. I was typing the word speedtest and the s would crash the keyboard and disappear. However the problem has seemed to go away. Which is odd. Since there were no software updates.