According to what you said, it should be a dent in the filling material. The occurrence of this situation should be related to whether the dental treatment is completed, the type of filling material, your own bite strength, etc.

Guidance:

Tooth treatment has not been completed. Temporary sealing materials may be used in the middle. It is normal for dents to appear. If the filling material is poor, saliva will corrode and dissolve and cause dents, so the filling must be refilled. ; If your bite force is strong, and the hard object happens to be located at the filling site, the depression caused by this will also need to be filled again.