Prior to the MegaCorp DLC in Stellaris, I would often declare war on an enemy AI empire and conquer systems I had laid claim to using the "Conquer" option, and I would notice the enemy's war exhaustion rising rapidly… far more rapidly than mine, as I would be occupying their systems by force, defeating their fleets and stations. I did this countless times in dozens of games all with more or less the same experience.
Fast forward to the MegaCorp DLC, and the exact opposite is happening: this empire, whose economy was 'Equivalent' and fleet power 'Inferior', somehow managed to not only constantly out-produce me militarily, but also enjoy a near 100% warp-out rate during every battle, even when the dialogue window during combat would show me destroying ships. These battles I won were also giving me about twice as much war exhaustion than the enemy AI received… which seems completely broken; before MegaCorp, I would receive almost no war exhaustion for battles I won or systems I claimed without losing any of my ships.
Is this some new Starcraft II-esque defensive cheat mechanism that was added to the game to make it more challenging, or a glitch, or what? This is a remarkably different experience from the dozens of other games I've played where I've gone to war. I can appreciate a tweaked AI for a more challenging and engaging experience, but this was… not that.
This opponent civ was a Constitutional Dictatorship & Harmonious Collective, with Fanatic Authoritarian and Xenophile traits, and I was playing on the difficulty above Ensign (what should not be that challenging of a difficulty), for what it's worth.
Best Answer
Yes, it seems that there is such "feature": AI fleets have a much bigger than expected chance to disengage
This can be further boosted using admirals with Trickster trait (+25% disengage chance) and the "Hit and Run" war policy (+33% disengage chance).
This can make wars longer and bit more difficult since you can't decide everything in one big battle. But there are few ways to counter it: devices that reduce the chance of disengagement (available on Titans and starbases), fighting in black hole regions and PROBABLY by increasing the size of the weapons (if you can take the enemy ship in 1-2 shots, they won't have time to disengage). You can also reduce the overall effect by trying to rush enemy ports