Skybox stetch

Hi.
Still studying how to use unity, and usually i can troubleshoot every problem i face but now i'm completely stuck and can't seem to find any mention of this problem anywhere. Phone build of the game that i'm making has a weird texture bug - skybox is stretched in the corners of the screen. What i managed to discover is that other 6 sided skyboxes from different asset packs all have same issue, but at the same time equirect skybox seems fine. Any idea why this happens?