Acne is caused by blocked skin pores. This can come from dead skin skills, oils, makeup products, dirt and bacteria.
Before the new normal of wearing face masks, your skin was able to breath easily and it was easy for dirt and dead skin cells to fall from your skin uninhibited. However, face masks have now made this impossible. Dirt, sweat, oils and makeup are instead held in place, clogging your pores leading to mask acne.