This was unexpected, and not documented at all in the SharePoint SDK.

When you create a UserMulti column, you must also specify Mult="TRUE", otherwise it will fall back to a standard (single select) User column type.

  <Field ID="{12312312-1234-1234-1234-123412341234}"

…
         Group="My Group"
         Type="UserMulti"
         Mult="TRUE"

…