I'm experiencing the same kind of game-ending retaliation from the AI time and time again. I tried 3717 yesterday and 3718 today, getting the same result: I hang around my base, assemble a fleet and fortify nearby wormholes, then lash out against the best looking nearby system in order to capture it. I am not entirely certain that the following behaviour by the AI is a reaction to my actions, but a 30-40 minutes or so into the game, the AI (diff 7/7 and diff 6/6) arrives with no mention of a wave or attack, with a huge fleet of mkII, mkIII and mkIV ships as well as a bunch of guardians and sometimes starships. They usually eat through my fleets and turrets in a minute or so.
I might have misinterpreted the changes and fixes report, but I am under the impression this was identified and fixed in today's beta, why I thought it worth mentioning.
Can confirm yet another game ending in 20 minutes, again shortly after I take the first system: Several mk III guardians entering my home system unannounced, then a bunch of normal mkII ships come along. At this point in time, the artillery mkIII guardian effectively reduced what was a seemingly well turreted system with an army of 100 mkI one ships to nothing.