graphviz appears to use a non-deterministic algorithm to optimally lay out notes optimally. This can appear in class hierarchy documentation, etc. Can presumably affect images of all complexity but, obviously, more likely to affect images with lots of nodes. . see https://twitter.com/Graphviz/status/1039632469782396929
Our notes about issues affecting packages are stored in notes.git and are targeted at packages in Debian in 'unstable/amd64' (unless they say otherwise).