Customer Support ETA unexpected automatic update

Hi,

for one of our customer (15.4 SU3), in Customer Support we have sometimes an unexpected automatic ETA update. In Back-Office, when the first activity is fullfilled with an incomplete Stop Code, the ETA Date of this activity is automatically updated, see the case below. Why do the ETA date changes ?  
Open_date :  15/04/2025 12:46 
Contr_ETA :  16/04/2025 18:00 (SLA next day + 10h)
ETA_Date :   16/04/2025 18:00
Coverage plan : each day 08h00 - 18h00 (duration 10h00)

We assign an AM and fullfilled the Activity (no more actions) in real time for instance on 15/04/2025 13:42 with incomplete Stop Code. 
Then, the ETA_date becomes : 15/04/2025 22:46 (10h00 after the Open Date)….

NB. If after this we enter a severity (Contract exception on this severity), then the ETA_Date is updated with a new value, and if we erase the severity then the ETA_Date becomes correct, 16/04/2025 18:00, as it was before we fullfilled the 1st activity. 

Thanks by adavance.