We no longer use Redactor, and we use an editor called Froala now.
Could you please attempt to reproduce the issue on this page:
https://www.froala.com/wysiwyg-editor/
We're also going to need very specific instructions on how to trigger it. I think I just repro'd it on our forum here, but after that it seemed to be ok.
Ideally, if you can provide the exact settings you're using on your phone, and list the exact keys that need to be pressed to trigger the issue, that will help us more consistently test it in future.