Hi Daniel!
You're welcome! Thanks for following up!
It looks like you are being affected by a known bug we are currently tracking in Storyline 3. This bug causes all text from a line to be deleted when a user clicks 'Backspace'. I've linked this discussion to the bug report and will provide updates as soon as we have news to share.
In the meantime, if you'd like to explore authoring in Storyline 360, another user shared a helpful workaround in a related discussion.
Thank you for your patience. Sorry if this has been slowing you down!