This is a weird one - at first I wasn't able to reproduce the issue, but then I figured out it only happens when trying to delete the leftmost digit if it would leave an exposed comma. It seems that I can backspace any other digit, including the commas themselves, and even highlight and replace the leftmost digit. In your proposed scenario, you would be able to edit the 4 just fine! It's only the 2 that would give you issues.
Good find! I'll let the team know about this. I don't imagine it will be high priority, but it will make a good issue for our ongoing UX/Quality project!