UPDATE: Looks like I have a GIT merge conflict message in my JSON file, that was preventing it from being loaded at runtime.
I’m seeing this same behavior on a “Page Link” field type, when I rely on the local JSON to load the field sets at runtime.
When the field groups are loaded from JSON at runtime, I get the ID of the page when using get_field. When I’m using a “synced” fieldset, I get the URL of the linked page.
It seems the issue is related to return format differing depending on how the field group is loaded.