Hi everyone, since I've made this silly mistake twice, I am typing it up for me and possibly others to find.
I called a .NET custom action using the predefined installer action.
Then when it ran, my CustomAction installer class code could not access the APPDIR path information.
The reason was that although I had added the /h="[APPDIR]\" to the 64bit install customization, I had not added it to the 32bit install!
AdvancedInstaller nicely automatically generates the two kinds, so be sure to edit both with your settings.