I can reproduce it every time, maybe I can even create a small demo file.
Whenever I open a solution, then quickly select another window (say safari) while filemaker is working with a script, when the script finally tells to open a card, it doesn't. The New Window step doesn't throw any error.
FileMaker Pro 16
OS and version: MacOS 10.12, 10.13
The only workaround is to NOT click on other windows until the startup script is finished, and then the card appeared.
That normal? I know that cards are still new, but the issue hasn't been solved, so I think it's time to report it.