Awesome glad you got to the bottom of it, and this will hopefully help others that run into the same thing. Thanks for sharing!
I just tested here and you get something like the following dumped out to the tk-desktop.log (I formatted it a bit to make it easier to read):
2019-09-03 08:44:17,896 [56862 ERROR sgtk.env.project.tk-desktop]
App configuration Error for tk-multi-launchapp (configured in environment '/sg_toolkit/configs/my_project/env/project.yml').
It will not be loaded:
Invalid configuration setting 'hook_app_launch' for tk-multi-launchapp:
The specified hook file '/sg_toolkit/configs/my_project/hooks/tk-multi-launchapp/broken.py' does not exist.
So in the future if anyone encounters similar behaviour, try searching the logs for something like “It will not be loaded”, and that should hopefully give some details on why an app is not loading.
Cheers
Phil