An update: I tried a new project with a different font, same sizes (76 and 96).
This time I created the 76 font first and tried it alone: it failed in the same way, i.e. on the older iOS devices! 96 later worked, as in the last example. So the title is wrong: some fonts are not working on old Apple devices, even by themselves (new devices were again ok).
Examining the fonts I saw that the bitmaps of the 76 fonts have a somewhat non-standard size: 512 x 768 (the bitmap for the 96 font is a more standard 1024 x 512). Could this be a problem?