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.