I tried adding !important, but to no avail. It's still an issue on mobile.
As an experiment, I deleted the custom font h1 font and instead used this code to specify another built-in font (Poppins):
h1 {
font-family: 'Poppins'!important;
font-weight: 300!important;
}
As with the custom font, it works on desktop but not on mobile. I am beginning to wonder if this is not font-related but a deeper issue with SquareSpace.
It shouldn't be this hard to use two different headline fonts.
Thanks for the note about Playfair Display.