This means one of:
1. In your interface at the stage there are 2 deliverynote fields and you overwrite the value
2. The TTN is created on another page and then the order is saved on the 3rd page, after which the TTN number is overwritten
3. The TTN is overwritten by an action or procedure when moving to a stage.
https://lums.crm-onebox.com/admin/shop/workflowstatus/158/action/new/ for example, here there is an action of copying TTN from a subprocess (disabled), perhaps something similar exists in procedures.
All 3 options are an error in the power supply settings, i.e. TTN is frayed precisely because of interface settings or actions