@Adrian Chirtoc Thank you for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
All the reservation recommendations for a particular service are calculated as shown below
Based on the information, I understand that for a particular app service you are not seeing any reservation recommendation in the azure portal.
Can you check whether the recommendation quantity for other resources in your subscription which exist from last 60days. if the recommended quantity is showing as 0.
Then I would suggest you use this Reservations Recommendation-List REST API and check.
If you are still facing any issues. please help us with the requested information over the private message so that I can check and assist you accordingly.