Handling application updates with App Installer and MSIX in Windows 10 19H1

I have a very strange problem attempting to update using app installer and I have no idea where to ask about it, or to report it as a bug except maybe here.

My application installs fine.  On updating it and launching it, the update is detected fine.  On clicking the button to install the update it fails with the following lines being written to AILog.txt

ERROR: StartInstallOperation -> Install Operation Failed: 0x80070057

GetErrorMessageFromHResult -> Message: [The parameter is incorrect.]

Watching the update process in Fiddler, I can see that the appinstaller file is read in its entirety numerous times and then two overlapping range requests are issued for a very small part of the msix package.  

I cannot understand what "parameter" can be incorrect since the application installs fine and the msix can be updated directly from powershell.

Rather I wonder if it is this strange range request processing that is buggy and the failure is in fact an internal one when it calculates its next wacky range.

Is there anything I can do to troubleshoot this further?