I recently updated to FM 15: (15.01.119), running on a Mac Book Pro with OS 10.11.16.
Open Type fonts use the open type features “mark” and “mkmk” to properly position diacritic marks (such as accents) with respect to base characters.
In FM 13 all such fonts that I have worked correctly.
In FM 15 none of them now work correctly. The on-screen appearance is highly degraded; accent marks appear on top of letters and on top of each other. It is a huge mess.
I can see no explanation for this retrogression in functionality.
I have also noticed that certain fonts which presumably require sophisticated open type feature support, such as Noto Nastaliq Urdu (a google font for Urdu) now appear completely garbled in FM 15.
In FM 13 running on the very same machine with the same OS, this font worked beautifully.
Compilation of screen shots given below, comparing exactly what I see on my screen in six cases, correctly in FM 13, garbled or incorrect in FM 15.