Hold on !
I was working on Firefox and out of an hunches I tried on Brave and it works !
Firefox version : 81.0 (64-bit)
Brave : Version 1.12.105 Chromium: 84.0.4147.105 (Official Build) unknown (64-bit)
So my iframes are not working on FF (I have all security modules deactivated on FF / Localhost)
Nope, I was wrong again, it was Ghostery that wasn’t properly deactivated for localhost:1313
So thanks a lot jmooring, our exchange solved my problem in a way 