Intellisense degraded when using devcontainer remote

Environment

OS and Version: macOS (Sonoma 14.6.1), Windows (10.0.19045)
VS Code Version: 1.92.2
C/C++ Extension Version: v1.21.6 (also tested 1.20.5, 1.16.3)
If using SSH remote, specify OS of remote machine: n/a
Devcontainer remote: Ubuntu/jammy, Ubuntu/noble, Debian/bookworm

Bug Summary and Steps to Reproduce

Bug Summary:

Intellisense is not erroring / reverting to tag parsing on devcontainer remotes. May be related to #12557, #12455. Let me know if you'd rather I add this context to a specific open issue.

Example minimum viable error at https://github.com/ianmclinden/vscode-cpptools-errcase

Steps to reproduce:

Open the example repo.
Under the host workspace, Intellisense works and the src/main.cpp:L9:WillReadMe shows errors squiggles. Autocomplete works fine.
Under the devcontainer workspace the error squiggles do not show up, unless intelliSenseCacheSize is set to 0. Autocomplete is degraded to tags.
Setting "C_Cpp.intelliSenseCacheSize": 0 masks the issue

Expected behavior: Devcontainer remotes should have working intellisense

Configuration and Logs

`c_cpp_properties.json`

{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "defines": [],
            "compilerPath": "/usr/bin/gcc",
            "cStandard": "c17",
            "cppStandard": "gnu++20",
            "intelliSenseMode": "linux-gcc-x64"
        }
    ],
    "version": 4
}

`C/C++: Log Diagnostics`

-------- Diagnostics - 8/20/2024, 7:20:22 PM
Version: 1.21.6
Current Configuration:
{
    "name": "Linux",
    "includePath": [
        "/workspaces/itest/**"
    ],
    "defines": [],
    "compilerPath": "/usr/bin/gcc",
    "cStandard": "c17",
    "cppStandard": "gnu++20",
    "intelliSenseMode": "linux-gcc-x64",
    "compilerPathIsExplicit": true,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": true,
    "intelliSenseModeIsExplicit": true,
    "compilerPathInCppPropertiesJson": "/usr/bin/gcc",
    "mergeConfigurations": false,
    "browse": {
        "path": [
            "/workspaces/itest/**",
            "${workspaceFolder}"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
cpptools version (native): 1.21.6.0
No active translation units.

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 2447
Number of files parsed: 1

Language Server Logs

c_cpp_lsp_log.txt

Other Extensions

n/a

Additional context

Host:

Note: autocomplete of the test namespaces also works, but is not shown.

Devcontainer:

microsoft / vscode-cpptools

Intellisense degraded when using devcontainer remote #12598