Including the --start-in-tray option now prevents Signal from launching. This is fairly recent breakage. I’ve been running this for over a year, but I noticed that Signal was no longer launching at boot.
After some experimentation, I can confirm that including the --start-in-tray option hangs launch.
Running KDE Neon version 5.18.2 (based on the current Ubuntu LTS 18.04)
Thanks,
Lance