Hey, thanks for the report. This is a known bug. The up arrow key in the chat input has been reported before in this thread: Up arrow isn't working in plan mode in chat box
The team is aware of this issue. Your report helps with prioritization.
For now, as a workaround, you can use the left arrow key, with Option or Alt to skip words, to navigate through your text.
Let me know if you run into anything else.