I have not yet tracked down the key element in the cause but the strongest contender appears to be having a custom domain.
When viewing a forum that includes Google fonts selected through:
Options > Application > Change appearance > Fonts
on a device that has the named font installed the forum correctly renders the text in the selected font.
On a device that does not have the font installed the text is rendered in Nabble's default (possibly the browser default. Sorry, not determined which yet!)
The forum has a number of sub-forums only some of which are subject to embedding and only one
https://www.stalhambaptist.org.uk/bulletin.php has embedding enforced (through "Redirect to:"). It is also the only page visible by the public.
On my desktop (Linux Mint) I have those fonts installed as I use them in documents I produce for the church, but on my phone or tablet I do not have those fonts installed and the forum appears to be rendered in the browser's default font on those devices (given that Nabble's default fonts are also not available on those devices) - so you could argue that Nabble's default fonts also require a review!
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.