[CSS] 2D skybox corners turn completely white when near edge of screen

[Ssig]

I am making a Counter-Strike: Source map. In-game, when I look up at the sky and move my mouse so that (what I presume to be) the corner of the 2D skybox is near the edge of my screen, it turns completely white. The angle I have to be facing for this to occur is quite precise, moving my mouse only slightly will cause it to go back to normal. It occurs facing different directions, but I must be facing upwards with the skybox towards the edge of my screen for it to occur. I have adjusted various HDR settings via env_tonemap_controller and the brightness of the white changes with them, so it appears to be lit.

The skybox is 64 units high along the top of all 4 sides of the map (4 objects) and the entire ceiling (1 object). I am using the cs_compound skybox that is part of the CSS assets.

I am compiling using the default compiler with all settings on normal and HDR enabled. The compile log says nothing about leaks and, apart from this problem, the map runs fine. There are no errors reported in the console and no FPS drops throughout the map.

I have tried removing various displacements, adjusting the skybox in different ways, and changing the light_environment/shadow_control/env_sun angles, but nothing is working.

I have not added any cubemaps yet, but I am not sure if that could be the cause. I was hoping to do those at the end and wanted to fix this issue before progressing further with my map.

Thanks for any help!

Screenshots:
https://steamcommunity.com/sharedfiles/filedetails/?id=2111919964
https://steamcommunity.com/sharedfiles/filedetails/?id=2111920109
https://steamcommunity.com/sharedfiles/filedetails/?id=2111920232
https://steamcommunity.com/sharedfiles/filedetails/?id=2111920482
https://steamcommunity.com/sharedfiles/filedetails/?id=2111920610
https://steamcommunity.com/sharedfiles/filedetails/?id=2111920783