I discovered I wasn't alone, as many other Linux users with Radeon GPUs and 16GB+ VRAM were experiencing similar problems. We created a GitHub issue to track the problem and try to find a solution: https://github.com/ValveSoftware/csgo-osx-linux/issues/2630
After some investigation, we found that Valve was punishing Linux users with certain hardware configurations (radeon cards with >=16GB of VRAM, which were quite new at this time).
Eventually, after a user reached out to gaben directly, the issue was fixed: https://github.com/ValveSoftware/csgo-osx-linux/issues/2630#...
I suspect this was because Valve was preparing to launch the Steam Deck, and gaben wanted to ensure that Linux users had better experience with the device (just a guess).