Well, within 20 minutes I already found a solution to this problem. I’m not sure if this is Webflow-side or Parabola-side, but there must be at least 1 product/cms item that populates this schema field in order for it to pull.
For example, if you add a new custom field called “mini image” to your product schema, there must be at least 1 product that has a “dummy” image set. If no dummy image is set, this schema field will not be pulled.
This still seems a bit odd, but it works.