It seems like this issue has to do with plugins that use GPU hardware acceleration, namely waves, fabfilter, and melda.
Cubase 10.5 was apparently built on Mojave which has depreciated OpenGL in favour of Metal which has caused plugins that use OpenGL and Cubase running previous versions of Mac OS to be quite unstable.
I’ve found that having a single plugin window open at a time seems to avoid the a few of the random hangs. Not all of course, but it helps. reduce say 10 crashes/hangs per hour to 5.