I get this on 18.04 with 3.1, but only on a slower machine with no nvidia graphics card. On a fast machine with 18.04 it works fine.
I used the source /opt/qt512/bin/qt512-env.sh trick and the extra ppa to get it working, so I figure there is not a lot of onus to fix this, but since others reported ame issue on supported platform.
I suspect its a timing issue due to the slow performance of my older machine where the issue occurred.