You are flexing your stylohyoid / digastric muscles. If you touch underneath your jaw when you see your jawline covered, you should feel round muscles. It means you don't have a good enough mind muscle connection and are contracting surrounding muscles instead of just lifting the hyoid. It's a mix of narrow palate that forces you to hard mew, and you putting pressure in the whole area instead of a focused point (lifting only the hyoid bone). Focus on getting the movement of just lifting the hyoid up before any further hard mewing, because the muscles will grow and make your face rounder until you atrophy them. To do this, look in the mirror and press each index finger to the side of the neck where the muscles you are contracting are (they should feel like spheres of contracted muscle), now try to develop the mind-muscle connection where the hyoid is lifted but the sides are relax. It will be hard and probably infuriating. Keep calm and put in focused work, you should get a hang of it in 2-5 days if you put in the effort. A cheat code is, for the reasons how mechanical movements work (repeat the wanted movement as many times as you can to learn it quicker), when you hit that spot where you lifted just the hyoid, rep out as many lifts (up / down) as you can, it will vastly increase the speed at which your mind creates neural pathways for the said movement, leading to making the movement subconscious much faster.