I see what you mean now. I had pasted your example into a note but not done the typing, or maybe I deleted the As and retyped them. I was confused by the example because it says to type As but already contains some.
I’ve edited your original post to fix the typo, emphasize that the span is deleted, and make the example less ambiguous.
I tried to make the example smaller but found it didn’t trigger the deletion. I didn’t explore to find the minimum number of letters required; maybe you already did that.
You may have found a bug. Could you file a bug report (if one doesn’t already exist), following the template that appears in the text box when you make a new post in Bug reports? (The template includes instructions for ruling out plugins and other non-Obsidian causes.)