[wix-users] Major upgrade requires reboot?

[Persson, Magnus (SE-TLX)]

By reading "WiX 3.6: A developer's guide to Windows Installer Xml" by 
Nick Ramirez I created a WiX installer with a GUI where I can select 
what to install and where to install it.

The installer project (made with VS2015) contains the below MajorUpgrade 
element.

<MajorUpgrade DowngradeErrorMessage="!(loc.DowngradeErrorMessage)" />

I did my install testing as a standard user in Windows 10.
One thing I noticed was that when I launched the .msi as a standard user 
two 'Msiexec.exe' processes were launched by my user. Why two?
When the GUI selections were done and the 'Install' button clicked, UAC 
kicked in promped for elevation. When I granted access with the local 
admin account, yet another msiexec.exe process launched (this time w/o 
name so I guess it's the local admin).

Everyting installed fine and on 'Finish' all three msiexec.exe processes 
terminated.

Now to the problem.
In the product element I changed the 'Id', change the 'Version' to a 
higher number (say 1.1.0.0 instead of 1.0.0.0) and kept the 'UpgradeCode'.
AFAIK this will result in a major upgrade.

The default behavior of a major upgrade is to uninstall after 
'InstallValidate'.

When I launch the new updated .msi I see two msiexec.exe processes by my 
name (same as before). One will allocate 1500K and the other 4500K memory.
As soon as the 'InstallValidate' is reached, a dialog with the text 
"Windows(r) Installer (Process Id: x)" with the buttons 'Exit', 'Ignore' 
and 'Retry' will appear.
The process id X will point to the msiexe.exe process that allocated 
1500K. Why is this process messing with the installation sequence?

If I select 'Ignore' the setup will complete but require a reboot on 
'Finish'. The major upgrade will succeed but why did it complain about 
the process that automatically launches?
How can I prevent the reboot requirement?

It seems that I am missing something.

Please help me.
Thanks,
Magnus