On FreeBSD 12 amd64 xfce-4.12 is installed, and octave-5.1.0 is installed. Applications menu shows the error icon for octave, even though it seems to have all the right icon files installed.
Works for me, octave-5.1.0 from Arch Linux repository and xfce4-panel 4.14.0. The icon is also correct in xfce4-appfinder 4.14.0. Unfortunately I wasn't able to build panel 4.12.0.
I know that sounds a bit rough but I will not investigate/try to fix this bug for 4.12. It may be in garcon or in the panel (probably related to the former), but it doesn't sound like low-hanging fruit and with the limited resources we have I'd rather properly support 4.14 and keep some time to work on 4.16.