Assertion failed '!"Missing BBF_PROF_WEIGHT flag"' during 'Transform async'

## Build Information
Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1590447
Build error leg or test failing: System.Xml.Linq.Misc.Tests.WorkItemExecution

## Error Detail
```
Assert failure(PID 6260 [0x00001874], Thread: 10168 [0x27b8]): Assertion failed '!"Missing BBF_PROF_WEIGHT flag"' in 'System.Xml.XmlWellFormedWriter:AdvanceStateAsync(int):this' during 'Transform async' (IL size 636; hash 0x5f302775; Instrumented Tier0)

File: D:\a\_work\1\s\src\coreclr\jit\fgprofile.cpp:4903
Image: C:\h\w\A8170911\p\dotnet.exe

----- end Thu 09/10/2026 8:03:42.56 ----- exit code -1073740286 ----------------------------------------------------------
```

## Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorMessage": "Failed test: System.Xml.Linq.Misc.Tests.WorkItemExecution",
"BuildRetry": false,
"ExcludeConsoleLog": false
}
```

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1590447
**Error message validated:** `[Failed test: System.Xml.Linq.Misc.Tests.WorkItemExecution`]
**Result validation:** :x: Known issue did not match with the provided build.
**Validation performed at:** 9/14/2026 2:05:58 AM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|

Start with the failing System.Xml.Linq.Misc.Tests.WorkItemExecution test and the build linked in the issue. Investigate the assertion at src/coreclr/jit/fgprofile.cpp:4903 during Transform async, then reproduce the failure if possible. Done means the assertion no longer occurs and the named test passes.

Written by the indexing model from the issue text.