Alright, we recently changed one behaviour which might cause this problem as a sideeffect, we'll look into that.
Are you guys talking about the situation when the NPCs are walking back home through everything? (buildings, unpassable terrain, plants, etc...)

On Hersfield there is a Patrician Filch spawn with 3 robots north-west of the station.

- sometimes they SPAWN inside a building
- sometimes if you try to get closer their optimal they try to retreat and walk into the building
- sometimes if someone walks outside this area and they follow him they get stuck when moving back to their spawn

It's easy to reproduce (the "stuck while trying to hold optimal" and "stuck while move back") if you like to check why/how/when.

PS:
It might be, that the fundament of the building isn't totaly set as "unpasseble" like
...#####
##........##
##.....X.##
...B####

The drone retreats after following someone to the X as it is "close enough to spawn" but then is stuck inside the building.

This would explain the "stuck on spawn" too as the spawn area might be to close to the building.

And the B might be the point, where the drones enter the building when trying to hold distance as there is a tiny tiny hole where they can pass. (I allready tryed to enter the building but my Arbalast seams to be to big).

changing it to
..######
########
########
..######
should solve the problem

That's not the issue so much, because when they're 'going home' they get a 'pass through anything' flag.  Unfortunately if you chase them and get within their aggression range they suddenly reverse course and sometimes they maintain that flag, pursuing you through buildings and over impassable terrain.  Sometimes that flag resets and they get stuck inside of buildings, but not impassable terrain which they just jerk back to the edge of.

Just turn that flag off entirely.  If players cannot decide they want to go home and ignore terrain/buildings blocking them then NPCs should be just the same.  If they get stuck, they get stuck, just like we do.
Also, set the NPC pathing flag to be a couple of seconds long rather than microseconds.  Currently when an NPC wants to navigate around something and move one direction the pathing flag abruptly resets and they go the other way (dancing NPCs).  Once the pathing flag sets a course let it remain that way for... say... two seconds so that the NPC can start navigating the path.

Also, look into the issue of NPCs shooting through buildings entirely.  This usually happens with triggered spawns as opposed to static.  I've lost count of the number of times I've dodged behind a large building to avoid an NPC only to have it continue blasting me through the building or hill.  This is rampant with Observers, less so with normal NPCs.

ah first forum search that got me a usable result :-)

I recently happened on a mech NPC (Artemis) with my Waspish and tried to take it down. I was in a quite complex landscape so was using that to shoot over hills and unpassable terrain. however the npc suddenly turned around and climbed a hill like it had jump jets or a jetpack installed. the trouble is, I could not pursue even if I wanted to (and finish him). he was out of range very quickly.

Stuck NPCs can be exploited (in my case, I am 100% sure I could have get it stuck and shoot it without issues), but seeing them go through anything is quite bad. Also just it seems not all NPC have the return home behaviour the same. From the same spawn was a mech digger that just stood his ground and was pounded to dust even when the  rest of the spawn went off my radar.