Simple answer: ended up being that I needed to register the VExpansionPanels component to the WrapperComponent like so:

const WrapperComponent = {
  components: { MyComponent, VExpansionPanels },
  props: { content: { type: String, default: 'text' },
  template: `
    <v-expansion-panels>
      <MyComponent :content='content' />
    </v-expansion-panels>
  `,
};