Comment # 8 on bug 1157921 from Wolfgang Bauer

Indeed, I updated firewall-applet to 0.7.2 (from the devel project), and I do
see the same problem now. Worked fine with 0.5.5 from the standard Leap 15.1
repos.

So it's certainly caused by some change in that package...

Maybe it sends the icon as pixmap data instead of an icon name? (this would
prevent the systemtray to adjust it)