We still see this occasionally. From a support ticket from IA, this is the solution that seems to work every time:
We've seen this issue get resolved by clearing the jar-cache files in the "data/jar-cache" folder in the Ignition installation directory. This would require a gateway restart, but you could get around restarting the entire gateway by deleting specifically the "com.inductiveautomation.perspective" folder if you're running Perspective or the "com.inductiveautomation.vision" folder if you are running Vision and then restarting the specific modules in the browser in Config > Modules on the Ignition gateway webpage. Restarting the module will require that you close and restart your current active perspective sessions/vision clients.
It seems that this problem only happens after we update one of our custom Python modules in the pylib folder. To mitigate this issue, we've set up our automation that deploys new modules to pylib to stop the gateway, copy the module to pylib, clear the jar-cache files, and then start the gateway.