If an attempt is made to remove contacts from an STO enabled list while a send is in progress, a warning appears.
Contacts cannot be removed if they are a part of a list or query that includes a send hour field as it places that contact on a waiting list, which prevents the contact from being manually deleted from Acoustic Campaign. Also, a recipient cannot be opted out or removed via the API while an STO email is still in the process of sending to that recipient. This behavior is as per design and cannot be changed.
This can also occur if current sends are in progress where the contact was included in the contact source used for the automated email that might still be active, such as auto-responders, automated messages, or programs.