السلام عليكم ورحمة الله وبركاته Recently, I've realized that when praying in a thawb, sometimes the thawb gets pinched between my buttocks, outlining that area's shape. Does this count as describing one's awrah, and does it invalidate my salah? Must I estimate and repeat prayers done in this thawb?

Ustadh AbdulAziz al-Haqqan

Actually, this is called waswas and you need to tackle this problem. However, since now you have become aware of this problem, you can fix it inshallah in your future prayers.