This has happened now twice while playing, once with the prior build of the beta and once again with the current version out for consumption.

Was playing a huge sized multi-star random map game in both situations and times it has occurred.   Normal settings across the board and with the other 9 computer players being set to normal with random AI.  Both times as the TEC Rebels.

The first bug was after several hours of having a relaxed, slow paced game of grinding through the XP to see just how good the various capital ships and titans could potentially be in a long enough game.  Suddenly, after a good deal of hours, the pirates stopped reacting.   They stopped moving and no longer were able to be targeted by weapons fire.   Any rebel ships that would arrive in a system would be bugged in the exact same way...  polluting systems with rebel ships that could not be removed.   The first time, it was after the pirate dens were cleared out so I thought that was the problem.   Second time, the pirate stronghold was still there...  jumped my fleet in to see what was going on.   Nothing moved in the gravity well.  The time was still in the corner of my screen, counting down to the next raid but it had no target.   This could be a bit of a problem if these ships do count towards system resources since they are still being rendered on screen and allowed to build up with no way to clear them out.

The second bug I noticed, or I think it is a bug, was that there was random generations of culture... rather overwhelming amounts of it actually.   Usually, this culture was of a color corresponding with the color of one of my neighbors I hadn't gotten to yet.   What made it annoying was...  instead of radiating out from their worlds, they were expanding from my own worlds that did not have a broadcast tower on it or were not next to a world who my culture had managed to propagated from yet.   Not only that,  I hadn't noticed because I had thought that buffer worlds along my border would keep enemy culture from penetrating into my core worlds who, due to needs of the early game for research and economy, no longer had the space for culture towers.

This has happened across several games...  as both TEC and Advent of all flavors.   When these have been noticed, they have been in periods before the Advent 'Joy Gun' super weapon had been built by any power...  often only being caught due to the usually last second warning of worlds about to fall to 'hostile' culture...   and deep within the interior of my empires.  

So far, this has only been noticed from Advent NPC factions...  but in one small scale 3 way game, it allowed one computer to overwhelm the other in quick succession due to the lack of slots for all sides to have all the needed buildings in a small game... and then it started snaking in past the few border worlds I could spare a tower or two to gut my interior.   It almost makes the game impossible or having to put a tower on every gravity well, just to keep the magic culture creep at bay.

Is this a feature?   Computer cultures being able to expand from worlds not bordering their empire?   Combine this with the random shut downs of pirates and rebels makes things difficult.