Shiny or reflective skin is due to an overproduction of sebum in the sebaceous glands of the skin. Shininess can also be caused by rich facial products that do not suit your skin type. In the summer when you sweat more, shiny parts of the face are more common, especially if you choose "wrong" products and makeup that form a membrane on the skin. If you use strong and dehydrating products that are not adapted to your skin type, the surface of your skin can become dry and the skin then tries to compensate for this by producing more sebum - which in turn means that the skin is fattier than should be. A daily moisturising skin care routine adapted to your skin type is important for keeping the skin and sebum production in balance.
For mixed skin it is common for the t-zone (nose, forehead and chin) to be shinier than the other parts of the face. This can be treated, for example, with a sebum-absorbing mask that is partially applied, and with a matte powder over the makeup that controls gloossiness during the day.