So, Wix (website building app) allows you to have courses, and courses can have members of staff assigned to scheduled sessions, and then the query API will return you course details including a staffMemberIds field so you can show who's involved in teaching a course on your webpage.

...except that said field is populated with all the staff members assigned to repeating scheduled sessions, and only to them. Obviously, if you only teach one session of a course, or the course only has one session you're not really teaching anything.

Hopefully this public service announcement will allow me to pay forward some of the three-ish hours I just spent tearing my hair out before I found this nugget in the docs, where it is just stated as if it is a completely sane and normal piece of behaviour for an API. The workaround? Make the single session a repeating session that finishes repeating before it's second scheduled date. That makes you a real teacher on the course.