We use a new UI lib called Jetpack Compose. Their keyboard doesn't seem ready for it.

Just an FYI GrapheneOS doesn't have its own Keyboard. The Keyboard that comes as default is the baseline AOSP Keyboatd. This seems like a very niche issue for it though, however personally I use Florisboard. Wh8ch is a candidate to replace the AOSP KB when it reaches a mature enough point and is feature complete.

As an update after doing some digging the issue was raised with Google and acknowledged relating to Jetpack Compose:

Google Issue Tracker

In the connected issues from that and connecting the dots they apparently fixed the problem with 1.3.0-alpha3 foundation unless I missed a dot.