I was experimenting with fake players when I realized that some of the players I spawned didn't have skins, I have the paid version of Minecraft. I went to NameMC and searched for the names I wrote. The accounts I searched for, had skins, but they weren't showing up in-game.
Now "RagePlaysGames" has a skin, I'm not sure if this is because there were 2 players with the same name or not.
The fake players that were supposed to have skins, also have lowercase names. I tried finding mods that could be interfering with the skins, but I didn't find anything.