This is a known bug that has already been fixed. When “Submit with Ctrl+Enter” is enabled and the “Try Plan Mode” suggestion appears, Ctrl+Enter incorrectly activates Plan Mode instead of submitting.
The fix is shipping in the next update. In the meantime, you can work around it by pressing Ctrl+Enter a second time after the nudge is dismissed, or switching to the nightly build where the fix is already live.
You can track the earlier discussion here: Submit with Cmd+Enter switches to Plan mode