I’ve activated the pro plugin and enabled the “Configure your site for the HSTS preload list” setting but alas my HSTS header is still coming up strict-transport-security:max-age=86400 which carries 3 failures, the max age is too low, preload, and includeSubdomains are missing.
I’m sorry, your post ended up in the spam, so I only noticed it now.
If you want to submit your site to the preload list, you have to enable a second option: the preload list option. This one will become visible when you have enabled the HSTS option. That will adjust the header so it will pass the preload list check.