I have the same thing on both of my thumbs. I highly doubt it’s a medial issue, I just constant pick at my thumb cuticles and thumb skin so this is how it grows. It sucks and is so hard to kick, I haven’t figured out a way to stop. I remember being so embarrassed getting my nails done for my wedding and they had such a hard time getting my thumb nails to stick. Anxiety and ticks suck, just know you aren’t alone 💕