I am glad you solved it but the mystery remains. Why did it reboot? Wix# doesn't provide and direct facility for rebooting. May be it even should... but it doesn't. So you couldn't do it by accident. After some googling I got an impression that the nature of the install may trigger it (e.g. installing drivers). Also it is evident that "...the installer automatically prompts for a restart if it needs to replace any files in use during the installation." and upgrade falls under this very scenario.
I would like to incorporate your work around into the codebase. Can you please tell me if making IsDeferred was required for your case. MSI documentation doesn't say anything about it but may be it doesn't work otherwise.
I would like to incorporate your work around into the codebase. Can you please tell me if making IsDeferred was required for your case. MSI documentation doesn't say anything about it but may be it doesn't work otherwise.