Steve
Mostly Harmless
The issue appears to be hidden tags. When you backspace into that, you've deleted part of the end tag. You can cap it with [/ quote] (remove the space in that) to get back outside the quoted info. Likely some part of the tag will show up in the quoted info - you should see that in your quoted material, as I replicated what happened and used the end tag to close it again.
It's always a little dicey to monkey with code when you can't see it. You can "toggle the BB Code" by clicking on the three vertical dots menu on the right of the screen, and then the bracket option. Then you can actually see the code and adjust it as you suggest.
For example, doing that, I could see why when I tried to reply to your post, because you included the code (/quote) in your post, it didn't display correctly. But that was easy to fix by toggling the bb code.