It's a feature for the citation style so you can change this either by switching to a different one or by modifying the citation style. Which one are you using?

You'd have to find
<layout prefix="(" suffix=")" delimiter="; ">
and change it to
<layout prefix="(" suffix=")" delimiter=", ">

See https://www.zotero.org/support/dev/citation_styles/style_editing_step-by-step for general instructions. Note that ; is correct APA style, though.