If you exercise and diet to control your weight, then it is likely that you are losing fat from your face. Weight loss occurs first from the face and then from other parts of the body.
Since the scales are showing that you are gaining weight, it is likely that you are gaining muscle mass and losing fat. Since muscle weighs more than fat, the scales may show an increased weight even when you are losing fat.
I hope this helps.
I am glad to inform you that there are no diseases that can cause weight loss from the face alone - no cancers nor any infections like HIV.
Loss of fat from the face can occur either because you are losing weight overall (which you are not) or due to sagging of muscles of the face. At your age, you should not really have much sagging of the face muscles.
But I believe that to prevent further thinning of the face, you need to go for a regular facial from a professional beautician. This will help stimulate the skin and tone up the muscles. You can even give yourself a light face massage once a week using a good massage cream or a baby oil. You can see the steps of a home facial massage here:
You also need to drink enough water - at least 8 glasses - as mild dehydration can cause the skin to sag and lose tone. A good multivitamin multimineral supplement. especially one containing Zinc will help.
Weight loss occurs first and is more noticeable on the face. Any HIV patient that you have seen who has a sunken face but have not apparently lost weight from their body would show a weight loss if their weight was measured. But this change shows up first on the faces. So this causes them to appear to lose weight on their face but not on their bodies.
If a bit of skin of an HIV patient who has a sunken face is picked up with two fingers, you would probaly be able to note that there is little fat under the skin, although they may appear to have no fat loss on the body.
I believe that you are worried that the loss of fat from your face could be probably due to an HIV infection. Let me assure you that weight loss in HIV occurs much later than other symptoms like diarrhea, cough, pneumonia, skin eruptions. In almost all patients , HIV is diagnosed much before the actual weight loss occurs.
HIV rarely, if ever, has weight loss as the first symptom.
Deficiencies and dehydration may cause loss of fat overall from the body and show up first in the face.