Hi Giulio.
It was a pleasure to get a reply from you. Apparently, I have contacted Apple Support via chat two times yesterday.
First in the morning, the Advisor (Jeffrey) diagnosed my iPhone and said the diagnosed test advised of an alert with the battery health and indicated the devices need to be serviced.
However, in the evening, I contacted another Advisor (Lee) and run the test again for my iPhone. And this Advisor said there is no problem with my iPhone.
Basically I have an appointment on this issue as the first Advisor booked for me and the second Advisor advised me to go also because of the report made by the first Advisor.
I am afraid that I need to bring my iPhone but there is nothing happen especially when I need to leave my phone at the service center.