Permissions problem with Wix Custom Action and Burn bootstrapper - permissions

Question

I have a package that needs to be installed with elevated administrator privileges. However, this installer includes an uninstall custom action to run a clean-up executable, which must be run as the local user.
This all works fine for the generated MSI (install and uninstall), but when I wrap the MSI up in a Burn bootstrapper, the uninstall custom action is instead run as the administrator - and hence fails...
My application must be installed “perMachine”, and so in my Product.wxs I have my package setup with
InstallPrivileges="elevated"
InstallScope="perMachine"
However, this application's uninstaller includes an uninstall custom action to run a clean-up executable, which must be run as the local user (since it looks for a file in the user’s App-Data). So I’ve set the CustomAction to
Impersonate="yes" Execute="immediate"
And have:
<InstallExecuteSequence>
<Custom Action="UninstallCleanup" Before="RemoveFiles">
(NOT UPGRADINGPRODUCTCODE) AND (REMOVE="ALL")
</Custom>
</InstallExecuteSequence>
This builds my MSI file, which I can then install on my machine. When I then run the uninstall, either via the Control Panel or manually (msiexec /x MyInstaller.msi /l* Testing.log), I am prompted for an admin password, the application is uninstalled, and the custom action is run as the local user. Everything perfect.
BUT I need to wrap this MSI up in a WiX Burn bootstrapper EXE that also installs pre-requisites (the VC runtime) before running my MSI package. Now when this Package Bundle is installed, and then uninstalled via the control panel, my MSI uninstall custom action is run as the administrator rather than the local user - and so fails.
In my Bundle.wxs file I have:
<PackageGroup Id="PP_Package">
<MsiPackage
Compressed="yes"
EnableFeatureSelection="no"
SourceFile="$(var.PP_MSI_PATH)"
Permanent="no"
DisplayInternalUI="no"
Visible="no"
Vital="yes">
<MsiProperty Name="INSTALLFOLDER" Value="[InstallFolder]" />
</MsiPackage>
</PackageGroup>
And
<PackageGroup Id="VCRedist">
<ExePackage Id="vcRedist_x86"
Cache="yes"
Compressed="yes"
PerMachine="yes"
Permanent="yes"
Vital="yes"
SourceFile="$(var.VCREDIST_X86_LOC)\$(var.VCREDIST_X86_FILE)"
Name="Redist\$(var.VCREDIST_X86_FILE)"
InstallCommand="/install /quiet /norestart"
Protocol="burn">
<ExitCode Value="1638" Behavior="success" />
<ExitCode Value ="3010" Behavior="forceReboot" />
</ExePackage>
</PackageGroup>
And then:
<Chain>
<PackageGroupRef Id="VCRedist" />
<RollbackBoundary Vital="yes" />
<PackageGroupRef Id="PP_Package" />
</Chain>
What have I got wrong? Is the issue something like: WiX Burn detects that the MSI is perMachine and so starts it using elevated admin, so that the custom action interprets Impersonate="yes" as meaning the administrator rather than the local user?
Any help, explanations and solutions would be most welcome.