Short answer
HubSpot's KB: each Brand can have its own resubscription email, turned on separately. If the setting was on before, it stays on only for the default Brand. Test emails always show the default Brand's unsubscribe information.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Turn it on for each brand you add, not just the default.
3. Related
Resubscribing contacts and subscription types.