Based on the several games I've played with 4.0, I'd have to agree with the OP that the AI tends to mostly behave the way they describe, though it is clearly dependent on the AI type too. However, I'd say it can't be called 'uncommon', because it happens more often than sometimes. Whether it's being caused by the AI type, ranged units shooting at everything or whatever, I do think it happens too easily, regardless. I'm sure upping the AI difficulty would make things harder, but what if I'm content with the difficulty of everything else I'm facing. Why would I want to up the overall difficulty just to make one thing more challenging and everything else too difficult. It's really more a balance issue, I reckon.
I dunno. I just feel like the AI has become a bit too predictable in these situations, and that makes the specific area in its behaviour weak and easy. Unless you are indeed facing planets with a ton of ships, you can always count on the fact that going in and out of the AI planet will cause it to send majority of its forces after you, to your turf where they are at disadvantage, instead of fighting alongside the planet's defenses, including guard posts. That's one reason why I'd personally prefer guardians don't exit the AI planet unless the command station is lost. That way at least some of its forces were to stay behind, even when using this tactic, even if it's just some guardians. And really, a dozen high-mark guardians is nothing to laugh at... until they follow your fleet to your defenses and you pummel them with twice the firepower.
I dare say, this behaviour IS common to all of my games, early and endgame, regardless of AI type, difficulty 7-7.6. Yet the difficulty isn't too easy for me, because I've lost games with the same settings. But offense... it's nearly always way too easy because the AI wastes its defenses with behaviour like that. If they were instead to pile their defenses at the wormhole you attacked from, but not send them through, THAT could make things a lot harder. Having to face everything at once, on the AI's turf, alongside any nasties the AI can also have, like ion cannons, OMD's, armor inhibitors/boosters, etc.
Also, I think there should be an option to do something like halve the selection of bonus ships both the player and AI get, to give those hull types and multipliers a bigger meaning. It's a thing that doesn't need to be awfully balanced, IMO. I've actually enjoyed the few times I've faced AI ships that have a hull type none of my mobile ships, sometimes even defenses, are particularly good against, as long as it's defendable somehow. The bigger the selection of stuff on both sides, the bigger the chances you'll have a counter for everything, and the less likely it is that the AI would have a lot of something you have nothing to overpower with.
