I wouldn't call it a bug.

Any software running in kernel mode needs to be designed very carefully, because any error will crash the entire system.

The software is risky because it needs to run in kernel mode to monitor the entire system, but it also needs to run unsigned code to be up to date with new threats as they are discovered.

The software should have been designed to verify that the files are valid, before running them. Whatever sanity checks they might have done on the files, it clearly wasn't thorough enough.

From my reading, this wasn't an unforeseeable bug, but a known risk that was not properly designed around.