The support for CodeMirror's vim mode is limited and feature set is spotty. To be fair, it was only allowed because CodeMirror includes vim mode, but it was never intended as a main driver.
Hence, most issues you might encounter in vim mode are not to going be fixed.
In the past, there were even requests to remove the vim mode completely because of this situation.
So, if you heavily rely on vim, you'd better off using it as an external editor.