Following up from the discussion in PowerShell/PowerShell#10537:

Currently, script blocks passed to Start-ThreadJob default to the process-wide current directory, as reflected in [Environment]::CurrentDirectory.

However, this is problematic, because that directory has no guaranteed or obvious relationship with the caller's current location.

Therefore, defaulting to the caller's current location instead is the most useful and intuitive behavior.

While technically a breaking change, my sense is that it falls into Bucket 3: Unlikely Grey Area, given that the effective $PWD is virtually unpredictable by the user:

$PWD is whatever PowerShell's startup directory was - irrespective of a -WorkingDirectory argument passed to the CLI - or whatever in-session code that changed the process-wide current directory last set it to.

A simple example:

PS> Set-Location /; pwsh -WorkingDirectory $HOME -noprofile -c 'Start-ThreadJob { $pwd } | Receive-Job -Wait -AutoRemove; $PWD'

Path
----
/
/Users/jdoe

That is, the above made / (or, on Windows, C:\) the $PWD for thread jobs, just because the pwsh instance was invoked while that directory happened to have been the process-wide working directory - even though the session's $PWD is $HOME.