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.
Discussion
As an update after doing some digging the issue was raised with Google and acknowledged relating to Jetpack Compose:
https://issuetracker.google.com/issues/227625842
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.