Test failure: System.Net.Security.Tests.TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain intermittently misses the peer intermediate on Windows x86 #136121

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1641185
Build error leg or test failing: net11.0-windows-Debug-x86-coreclr_checked-Windows.10.Amd64.Open-System.Net.Security.Tests.TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain
Pull request: #136040

The same failure also occurred in https://dev.azure.com/dnceng-public/public/_build/results?buildId=1640916 (PR #135067, a CI-only change), in the same Windows x86 checked library-test leg. It appears intermittently, in 2 of 177 runtime builds since 2026-10-12. The assertion that fails is Assert.True(callbackSawPeerIntermediate): the external validation callback's chain.ChainPolicy.ExtraStore did not contain the client's intermediate certificate. That is the symptom #134567 described, which was fixed by #134570, the PR that added this test.

Error Details

System.Net.Security.Tests.TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain [FAIL]
Assert.True() Failure
Expected: True
Actual:   False
at System.Net.Security.Tests.TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain() in /_/src/libraries/System.Net.Security/tests/FunctionalTests/TlsSessionTests.cs:line 564
--- End of stack trace from previous location ---

Error Message

{
  "ErrorMessage": "TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain() in",
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=1641185
Error message validated: [TlsSessionTests.ServerSession_ExternalValidation_PreservesClientCertificateChain() in]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 10/20/2026 1:36:56 AM UTC

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