How to resolve “operation could not be completed because the vhdx is currently in use” for wsl despite prior `wsl.exe –shutdown`?

I get an:

> wsl.exe --manage Debian --set-sparse true --allow-unsafe
Conversion in progress, this may take a few minutes.
The operation could not be completed because the vhdx is currently in use. To force WSL to stop use: wsl.exe --shutdown
Error code: Wsl/Service/ERROR_SHARING_VIOLATION

that persists after a:

wsl.exe --shutdown

I even tried to nuke wsl and related services:

Stop-Service vmcompute -Force
taskkill /F /IM vmwp.exe /T
Stop-Service vmms -Force

all to no effect. How to release the lock?