I am using an iPhone 8 running iOS 11.3.1.
I find the memory warning!
Appears after typing on the custom keyboard for long periods of time, but I have never seen it in the stand alone App, which I have used for hours at a time.
Re-booting the iPhone seems to clear the problem until I type for another long period of time.
When the warning comes up, I can flick up and see the word I tried to type, until I keep typing, then the memory warning will appear and not go away.
I hope this makes sense,