I’m not sure if this is known or not.
under Customize > Layout > Header –
Mobile Header = On
Branding Type = Logo
I’m using the same logo that is on the main site and its https – but anytime I use this logo here the cert gives an error.
So you can see I’ve added the same logo i’m using on desktop.
If you view source you’ll see the image is displaying as http:// and not https:// and the secure icon at the top has a caution sign.
If I remove the image and add text the site becomes fully secure again.