Signal-desktop: multiple instances with tray-icon?

I’m trying to run 2 signal-desktop instances minimzed to a tray-icon. Strangely, the second started instance ignores the --system-tray parameter and starts only as a window and quits when I close it.
I’m starting them like this:
signal-desktop --system-tray --user-data-dir=.config/Signal_instancename

I believe this worked on my Manjaro KDE installation. Help please!