Probably the font has been embedded on the paragraph level.
The simplest way is to use KEPUB instead of EPUB - either with the help of Calibre or Kepubify (
https://pgaskin.net/kepubify/).
Other ways are converting from EPUB to EPUB with Calibre and stripping out all font-family attributes in the process, or edit the EPUB manually with Sigil or Calibre editor.