Broken characters in iBooks

This happens when my client opens her FXL e-book in iBooks on her macOS machine.
The e-book is created in Sigil on the Windows 10 platform.

Is it:

• A conversion issue between Mac and Win?
• A broken font file on the client's computer?
• An issue with character encoding? (Currently UTF-8)
• A bug in iBooks?

Any suggestion to solving this issue is appreciated.