Can anyone confirm if this is this is meant to behave like this or if this is a bug?
Ideally If an email is marked as "operational" then it ignores the Unsubscribed and Marketing Suspended statuses. They send the email no matter what!
The only exception is Operational emails are not designed to work with engagement programs. Thus, an engagement program will ignore an email's operational status. But apart from that in case the operational email status is being ignored by Marketo then there might be a possible issue and a support ticket would need to be raised to resolve this!
Also you can go to the activity log of the lead and see the details of Skipped sending the email to confirm the reason of not sending the email!
Check the settings for your Sales Emails in Admin. You may have them set to Respect Unsubscribe Settings