C. B. Zeller, thanks for posting. I had heard that filing on time invalidates the extension several times but always wondered if it was an old wive's tale or whatever you want to call it.
I am curious if anyone has a cite for the opposing view.
Seems like your situation is similar to the one described in rev. rul. 66-144. The IRS ruled that the extension was valid for purposes of extending the deadline to make a plan contribution.
https://www.taxnotes.com/research/federal/irs-guidance/revenue-rulings/rev.-rul.-66-144/d5wt