Using 18.04 with Mutiny layout.
The Caja icon does not appear in the launcher. I can still get to my files using the “Home” icon on the desktop, but it’s annoying as it often gets covered by other windows.
The icon still doesn’t show in the launcher when Caja is open, so I often use Alt-Tab to get to an open window. Again, an annoying workaround.
I frequently reset using MATE Tweak, which sometimes solves the problem, but inconsistently and not for long.