Debugger: Mismatch value between data tip and local window after editing a variable in watch window

## Environment data

`dotnet --info` output:
VS Code version:
C# Extension version:
## Steps to reproduce

1 Set breakpoint at line of var cacheKey = "topselling" in HomerController.cs
2 Press F5 to start debugging
3 Change to watch window, add expression of cacheKey = "Hello"
4 Press F10 to step over to next line, observe the data tip of cacheKey and same variable in local window.
5 Press F10 to step over to next line
## Expected behavior

In 4th step: The cacheKey's data tip and local window should be the same value
In 5th step: The cacheKey's data tip and local window should be "topselling",rather than "Hello";
## Actual behavior

They actually have the different values In 4th step