Installed the Simple Keyboard open source app. Problem gone. 👍
Discussion
It's not the GrapheneOS keyboard it is simply the default AOSP keyboard that was forked to create GBoard at which point Google stopped updating it properly.
Any AOSP based OS which includes this has the issue as it is a conflict between it and Jetpack Compose used to provide the text input field.
Many GrapheneOS users use Florisboard or GBoard with network permission revoked or you can use any other FOSS keyboard to replace the stock one.
However if you use a password not a pin for device access make sure it works BFU (Before First Unlock) otherwise you'll cripple access if it doesn't and it is made default.
It doesn’t seem to have this behaviour on apps other than Amethyst for me 🤷🏼♂️ is that not your experience?
That is my experience however there are other apps out there with it and there are plenty of discussions related to it on the Google Issue tracker. Alas the fixes appear to have been focused on GBoard over the baseline AOSP keyboard.