Good point, and thanks for bringing it up.
Just to highlight: different VLAN IDs between Security Groups on shared uplinks are not only best practice, but a mandatory requirement. The system won’t allow the same VLAN ID across multiple SGs on a shared uplink, so this needs to be accounted for during design.
That said, the stronger recommendation is to use dedicated interfaces per Security Group whenever possible. Shared uplinks can create dependencies where external issues affect every SG tied to that interface, reducing isolation and resiliency.
Your example is a good reminder that planning these details upfront helps avoid costly redesigns mid-migration.