During a Rhamus Levi hunt I found an observer that could be damaged to 0% but would not die. Full ragloads hitting this 0% zombie could not kill it. There was no sector lag, damage beeped as usual, several attempts by several people were made.

Also, the client has been crashing to desktop regularly since this last update.

I haven't experienced any crashing.

Conflict Diamond, try running without any plugins to see if you still experience crashes.

I have seen multiple observers do this, however, have not experienced a crash yet. I can only suppose that the observer gets stuck in the sector when its health gets reduced to 0, but it fails to despawn... it just continues in a straight line along its present course, and nothing can damage it.

The Milanar and Siepos hives seem to have this problem, and I have yet to see an optics drop from either. Prosus hive drops them, and haven't encountered the evil undead zombie observers up there. Has anyone seen Milanar or Siepos optics drop?

Is this in any way related to the fact that only the Prosus Optical Sensor array manufacturing mission is showing?

Yeah. Milanar and Siepos Observer Optics should be dropping now. Thanks for the report.

could that be related to the time i got down to -1544% armor and didn't die, just sat there

Probably not.

I got 8 Milanar Observer Optics from ~20 Hive Skirmishes during the weekend,
so they drop quite often.

And I've seen a lot of Zombie Observers.