I have a fresh install of manjaro and icons started disappearing randomly. Weirdly enough this happened with one application on my previous machine but now this is just very annoying, every icons are missing from dolphin, and tons of application icons too.
It seems like you’re usind hicolor icon theme, try changing icontheme in systemsettings.
If it don’t work try removing ~/.cache/icon-cache.kcache file and reselect icontheme.
For best test try with breeze icon theme first.