No it was not a non-brand icon (but that’s good to know because I did want to use one of those!)
I think it’s a bit more complicated. When I selected the theme for customize without applying first, the “Social Links” drop down had one “pre-expanded” entry to type in. That one never shows up.
Once I _published_ the theme first, it would then not have this one “pre-expanded” entry, and all those added would show up fine.