Foreign gift tax on 1040 extension -- problematic?

A person filed for a 1040 extension on May 14, 2021.  He paid taxes on the extension later, on May 18, 2021.  

I see many people on the internet reporting problems when filing form 3520 after April 15, even after an extension.  The penalty for late submission seems to spew out right away, even with an extension.   Seems to be on auto-mode.  And then you need to appeal, etc.   The person has over $100K in foreign gifts and should file the 3520, but is concerned about the auto penalty deal.  

Does anybody have had experience with this?

Thanks