Does a man's prayer become void if what is above his knee is uncovered during prostration due to spreading his legs, knowing that only those behind him can see the uncovering of the 'awrah?

If the awrah is covered such that nothing of it is exposed, but it is visible from beneath the garment, this does not matter. This is based on what An-Nawawi stated in Al-Minhaj: "It is obligatory to cover its top and sides, but not its bottom." And Al-Khateeb Ash-Shirbini said: "Even if one were to see his awrah from its bottom edge, such as when he is at a high place and the viewer is at a low place, this does not matter." Therefore, your prayer is valid.