Wrong settings for config "Audit checks - extensions only"
the default configuration entry "Audit checks - extensions only" enables more than extension-provided checks, which is more than surprising (and very disturbing).
Go to the menu bar, then select "Burp > Configuration library"
Highlight "Audit checks - extensions only" and click on "Edit"
Go to "Issues reported", sort on "Enabled"
Two extra entries ("HTTP Request Smuggling" and "Backup file") are enabled
Tested on Pro 2.1.04
Thank you for your message.
I have repeated the steps that you have listed and am seeing the same results.
I have logged this as a bug request with the developers and we will notify you when it gets fixed.