Hi Eric,
Even I am able to reproduce it intermediately. here is scenerio by which you will able to see weird behavior:
copy the following html(with <b> tag:
You are subscribed to the following lists. If you would like to remove yourself from a list, uncheck the box and click <b>Submit</b> to update your profile.
Now place cursor at starting position before "you" and press Enter key twice then the it showing weird behavior.
2. other scenerio for #1 above(Firefox), I have copied the below text from email to notepad and then to editor
Approve Action Link {[R-APPROVE ACTION LINK]}
Approve Action URL {[R-APPROVE ACTION URL]}
Decline Action Link {[R-DECLINE ACTION LINK]}
Decline Action URL {[R-DECLINE ACTION URL]}
Now put the cursor at starting and press enter then the cursor move far below in editor without changing anything. I am able to reproduce it in your example too.
Please let me know if you need more information.