Your system information

• Steam client version (build date): 2021-04-12 18:51:43
• Distribution: Gentoo
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

$ lspci | grep VGA
29:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Navi 14 [Radeon RX 5500/5500M / Pro 5500M] (rev c5)
2a:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Ellesmere [Radeon RX 470/480/570/570X/580/580X/590] (rev cf)

$ glversion 
4.6 (Core Profile) Mesa 21.1.0-rc2
Radeon RX 5500 XT (NAVI14, DRM 3.40.0, 5.11.16, LLVM 11.1.0)

The display is connected to RX 5500. No display is connected to RX 570.

Comparing the sizes of foz_cache.foz:

$ cd shadercache.....Rise of the Tomb Raider 
$ tree --size
.
├── [4.1k]  DXVK_state_cache
├── [4.1k]  fozmediav1
├── [4.1k]  fozpipelinesv5
│   ├── [ 13M]  replay_cache.2f695f6084e13746669c35859ef7fd5f.foz
│   ├── [5.7M]  steamapp_pipeline_cache.foz
│   ├── [1.1G]  steam_pipeline_cache.foz
│   └── [ 16M]  steam_pipeline_cache_whitelist.foz
├── [4.1k]  mesa_shader_cache_sf
│   ├── [4.1k]  ce989c92dc1fbf9fa80ede9c4b68b3f9
│   │   └── [4.1k]  AMD RADV NAVI14 (ACO)
│   │       ├── [  16]  foz_cache.foz            <<<<<<<<<<<<<<<
│   │       ├── [  16]  foz_cache_idx.foz
│   │       └── [1.3M]  index
│   └── [4.1k]  f296f506481e736466c95358e97fdff5
│       └── [4.1k]  AMD RADV POLARIS10 (ACO)
│           ├── [199M]  foz_cache.foz            <<<<<<<<<<<<<<<
│           ├── [5.0M]  foz_cache_idx.foz
│           └── [1.3M]  index
├── [4.1k]  nvidiav1
└── [4.1k]  swarm
    └── [4.1k]  steam_shader_cache
        └── [1.1G]  bucket_4915160.swarc

Steps for reproducing this issue:

1. Install two discrete AMD GPUs
2. Upgrade Mesa libraries (which will trigger a rebuild of Mesa caches in step 3)
3. Start Steam with the option "Allow background processing of Vulkan shaders" enabled