My guess is you have updated your MacOS version recently. Unfortunately, that update includes fonts that have the same name as your old local font files you installed sometime (years?) earlier.
This font duplication breaks things as one font uses full unicode mappings and the older local version does not. The solution is to open FontBook and since you can not disable Mac OS installed fonts, you must disable the versions of your locally installed fonts.
See this issue:
https://github.com/Sigil-Ebook/Sigil/issues/831
Ps. This exact same thing happened to my Mac and it took forever to track down the reason, but disabling my local font files using Apple's FontBook app is easy.