When installing IoT Enterprise, however, the Insider Preview version with a watermark is installed.

Hi, the watermark indicates that preview media was installed; it does not by itself confirm the intended IoT edition. Run `winver` and check Settings > System > Activation to confirm the exact edition and activation channel. Surface Laptop SE firmware may also supply an embedded Windows SE key, which Setup can prefer. For IoT Enterprise, use the correct official IoT/volume-license media and verify that your organization has the matching license before deployment. If Setup keeps selecting the embedded edition, your deployment team can apply the correct image index directly. I would not try to convert a preview SE installation with unofficial keys.