The symbol server is missing symbols for Windows 10 Build 21296. symchk dump below.
symchk.exe /if build/windows/explorer.exe /v
[SYMCHK] Searching for symbols to V:\Temp\21296\build\windows\explorer.exe in path SRV*V:\Symbols*https://msdl.microsoft.com/download/symbols
DBGHELP: Symbol Search Path: SRV*V:\Symbols*https://msdl.microsoft.com/download/symbols
[SYMCHK] Using search path "SRV*V:\Symbols*https://msdl.microsoft.com/download/symbols"
DBGHELP: No header for V:\Temp\21296\build\windows\explorer.exe. Searching for image on disk
DBGHELP: V:\Temp\21296\build\windows\explorer.exe - OK
SYMSRV: BYINDEX: 0x1
V:\Symbols*https://msdl.microsoft.com/download/symbols
explorer.pdb
C771F9CAE89E3949F75BD3CA172F60ED1
...
SYMSRV: HttpQueryInfo: 80190190 - HTTP_STATUS_BAD_REQUEST
SYMSRV: HTTPGET: /download/symbols/explorer.pdb/C771F9CAE89E3949F75BD3CA172F60ED1/explorer.pdb
SYMSRV: HttpQueryInfo: 80190194 - HTTP_STATUS_NOT_FOUND
SYMSRV: HTTPGET: /download/symbols/explorer.pdb/C771F9CAE89E3949F75BD3CA172F60ED1/explorer.pd_
SYMSRV: HttpQueryInfo: 80190194 - HTTP_STATUS_NOT_FOUND
SYMSRV: HTTPGET: /download/symbols/explorer.pdb/C771F9CAE89E3949F75BD3CA172F60ED1/file.ptr
SYMSRV: HttpQueryInfo: 80190194 - HTTP_STATUS_NOT_FOUND
SYMSRV: RESULT: 0x80190194
...
[SYMCHK] PDB: ""
[SYMCHK] CV: RSDS
[SYMCHK] CV DWORD: 0x53445352
[SYMCHK] CV Data: explorer.pdb
[SYMCHK] PDB Sig: 0
[SYMCHK] PDB7 Sig: {C771F9CA-E89E-3949-F75B-D3CA172F60ED}
[SYMCHK] Age: 1
[SYMCHK] PDB Matched: TRUE
[SYMCHK] DBG Matched: TRUE
[SYMCHK] Line nubmers: FALSE
[SYMCHK] Global syms: FALSE
[SYMCHK] Type Info: FALSE
[SYMCHK] ------------------------------------
SymbolCheckVersion 0x00000002
Result 0x00010001
DbgFilename explorer.dbg
DbgTimeDateStamp 0x00000000
DbgSizeOfImage 0x00000000
DbgChecksum 0x00000000
PdbFilename explorer.pdb
PdbSignature {00000000-0000-0000-0000-000000000000}
PdbDbiAge 0x00000000
[SYMCHK] [ 0x00000000 - 0x00010001 ] Checked "V:\Temp\21296\build\windows\explorer.exe"
SYMCHK: explorer.exe FAILED - explorer.pdb mismatched or not found
SYMCHK: FAILED files = 1
SYMCHK: PASSED + IGNORED files = 0
