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.
With the move to https for my forum, it appears that no fonts are available at all due to the calls being made to Google Fonts API (Web Fonts) over http. This will result in those calls being blocked (mixed content) as they required to be made over a secure connection (https) as well.
What you end up seeing is the browser default font.
I actually even doubt that Nabble makes use of custom (named) fonts, which should be available on the server itself, with a reference to the location (@font-face). I could have missed it while going through the several sources, but to me it looks like only Google Web Fonts are being used, located in 'GWF\', with the option to choose any other Google Web Font available.
The use of Google Web Fonts is fine, as they are highly compressed and downloaded fast on demand. Then they are stored within the browser, for use elsewhere too. Just too bad they will be blocked when you have switched to https (mixed content), because calls to them are still using http.
I hope Nabble will fix this soon, which is easy, as there are only a couple of locations where they 'forgot' to change http to https.