Adam,
Please follow these steps below to reproduce:
1. Insert red html { <span style="color: red">Hello world</span>} into CE with HTML mode.
2. Switch to Normal mode
3. Delete all texts by pressing on Backspace in end
4. Enter any word like: ABC
5. Result <p> tag is insert automatically: (see it in HTML Mode)
<p><span style="color: red">ABC</span></p>
I can re-produce this issue on demo site.
Nothing's impossible, Nothing's unreachable.