I have noticed that since I recently installed Lazarus from SVN rather than my package manager, I see a generic icon in the doc (Ubuntu Mate 18.04).
I reckon this is because that build (as me rather than root) did not put icons in /usr/share/icons/hicolor/* Lets try and do that manually ....
Nope, needs more than putting images/ide_icon48x48.png into /usr/share/icons/hicolor/48x48/apps/lazarus.png - might need a desktop file too.
Further research is indicated ....
Davo