This happened after an update went wrong and when I restart my computer suddenly some files have an icon and others do not. It does not only happen with Dolphin icons, but also with other elements of the system.
Going to Settings and resetting the theme usually fixes this, but at random intervals it happens again.
what happen if u right click on one of the invisible icons, choose properties, click on “change” button then assign the icon manually from there. (you click on the button to the left of the extensions list)