What's happening?
The queue entity is not available in the "To" field on the main form of the email entity, although it is already present for the "From" field.
Reason:
This was caused by a custom script interfering with the visibility of the queue entity in the email form field.
Resolution:
Disable the customization involving the custom script. After disabling the customization, the queue entity will appear correctly in the "To" field of the email entity form.