Mark,
From what you are describing for mobile is a long page load that the browser sticks to a system font. fallback or optional does this.
Improve viewport load times or change the font display to swap or block.
I did a test with a new blank site and just used an H1 header as the only element on the page, I uploaded the font again to this site and the same issue appears. I change the font some other custom font and the problem goes away. I can only imagine there is some issue with the font itself
Well strange,
I don’t think there is any need not to load just woff2 so I would try that.
If you haven’t got the woff2 option convert one of the others say ttf to woff2.
Remove all but woff2 as one of the others could the issue. Although I would have thought the browser would load that 1st by default.
I am going from maybe one is not quite right when it gets to mobile.
If after woff2 you get the issue remove that and try woff then tff etc.
Something worth a try.
I got it working in the end with your suggestions. It isn’t 100% as I have to play around with the different Webfont formats to see which one is playing nicely, but your help has got me over that hurdle. Thank you