Usually fonts are changed if the font can't be found on the computer.
This may also be related to the theme you used, there are a few themes that have been deprecated since fmp12.
I would suggest reinstalling the font to verify that the font is not corrupt.
Fonts have always been on the computer. It may be that Filemaker doesn't know how to access them, as they are loaded via the font manager Suitcase Fusion, rather than through the Apple System manager. The fonts are fine.
I can't speak to the theme issue... It's a very basic layout, and was done way back (Filemaker in 2003 or 2004) so does it have a theme? Couldn't tell you.
Themes were added to FM with FMP 12.
If FM can't find the font then FM will substitute a different font.
12 converted all existing layouts to Classic Theme during conversion. 13 supported Classic, but 14 and 15 no longer support Classic Themes. Jumping from 12 to 15 basically abandoned obsolete theme support.
Time to assign new themes wherever old layouts were simply converted from pre-12 layouts.
Yes, maybe it’s time to give this solution a full revamp. There really was no theme at all when it was originally designed. It’s gotten unwieldy and lots of chaff in to the file.
One big issue is going from a custom font to the FM Go app — it doesn’t map to mobile very well at all.
I did find sort of a work-around. I moved the font in use to the System Font Book to permanently keep it in use, rather than activating via Suitcase Fusion. That fixed some of the problems, but not all.
Thanks for your input.
There are several fonts that are not compatible with FMGo. There was a list in the old FMGo development Guide, but I did not see the list in the 15 guide. The 15 guide did state that the font that most theme use Tahoma is not support on ios and would be converted to Helvetica. Much testing will need to be done and I would strongly suggest using a different layout for ios devices. FileMaker Go 15 Development Guide