The apt-packaged version of OpenSCAD was broken for the current LTS version of ubuntu and hasn't been fixed yet. You'll need to download it from an alternative source if you want it on 18.04. Works well on 16.04 though!
Perhaps I'll just use AppImages like you so that I can finally upgrade off 16.04. Have you found any downsides to using the AppImaged version over the apt one? (aside from the stability issues in newer releases)