I've seen it happen VERY early in Rebellion though.

The last game I played, the final AI surrendered with 14 capitals, a lvl8 titan, and several star-bases still left. Now could I have killed him, no doubt (max fleet with 14 lvl10 caps + lvl10 titan). But if he'd parked all of his capital ships, his (loyalist none-the-less) titan, and the rest of his fleet all on top of a star-base (or two even being loyalist faction) he surely could have made it a hell of a final stand against even my armada.

I hadn't even sent a single ship into one of his gravity wells yet when he surrendered. That seems somewhat pre-mature with what he had remaining. (And no way to know what I had, I was still far enough away he couldn't see my ships jumping)

Ya. the AI does surrender. sometimes before it should. sometimes way later.

It would be a nice-to-have if the AI instead of immediantly surrendering, would ask the player 'do you accept my surrender' Like in starcraft 2.

Really odd that you all are experiencing this since the update a couple of days ago.  After the update, I started a new game against an aggressive AI, and it fought tooth and nail up to the very last planet.  I would imagine it would have continued to fight even further if my culture hadn't neutralized its culture on the last planet.  The last hour or so of the game was spent with me sending a fleet in along with my titan while my backup fleet stayed behind to shore up the defenses on the newly acquired planets.  The titan would work over the starbase while the fleet took care of the other defensive structures and bombed the planet into submission.  The AI would send usually at least 2 waves of fleets to dislodge me while I worked over the planet and SB, sometimes with enough ferocity to actually push me back to the last star system I had just taken.

At any rate, it took me quite some time to push the AI back to its home world, then it kind of fizzled out due to my culture over powering it.

Part of it might be the classification of the titans at the current moment. Unless the AI is coded to see the titan as the powerhouse it is it might just regard it as another frigate or cap ship.