Sounds almost like a locking issue / racing condition then. Almost looks like
we have to try and compile mesa ourselves to figure out which patch to 18.0.1
that broke it and why 18.0.3 fixed it while 18.1.0 broke it again even though
18.1.0 contains all the patches from 18.0.3...

Currently I'm using the Padoka PPA but the stable one seams to be stuck at
18.0.1 for 16.04LTS and the unstable one seams to have dropped support for
16.04 altogether.