Update: July 29:
Maybe I spoke too soon. Today, I had a problem on another post even in Times New Roman when two paragraphs accidentally ran together when my finger slipped on the laptop touchpad. When I separated them, the second paragraph still got an unwanted white background from Blogger. This time, I deleted and re-added the second paragraph, and the white background went away without going into HTML mode to hand edit, which can lead to the Btx errors (in my experience).