Hello and welcome to Advanced Installer forums,
First of all, thanks for the provided files. I have tested this and I was indeed able to replicate the behavior.
At a first glance, this seems to be caused by an issue in what regards our "LaunchFile" custom action support. I have forwarded this to the development team and we will further investigate this. Hopefully, this will be fixed in a future version of Advanced Installer. Unfortunately, I can not give you any estimation on when this will be done. Thank you for your understanding and for bringing this to our attention.
As a workaround, you can add the .BAT file as a temporary file in your project. Unlike Advanced Installer's regular files or resources, "Temporary Files" are copied on the target machine at the beginning of the installation and are deleted at the end of it, ensuring you have access to them throughout the entire install process. Temporary Files are copied during the "CostFinalize" standard action, at the beginning of the setup and removed during the "InstallFinalize" standard action.
In order to add a temporary file to your project, simply go to "Files and Folders" page and press the "Add Temporary Files" button from the toolbar. After doing so, the .BAT file can be referenced in your "LaunchFile" custom action as it follows:
Hope this helps somehow.
All the best,