I'm seeing a similar, though slightly different, trait in the AI right now as well. Up until the most recent planet (asteroid, really, but whatever) I went after, the defending fleet would run home to the next planet in line, so to speak. I finally hit thier Homeworld, and the AI pulled a new strategy on me... it was Advent, so it made use of thier 'temporary invulnerability' powers from thier cap ships (the actual name escapes me - I'm more of a casual player) to basically keep every one of it's fleet from being damaged. There were six Capships of three different varieties, so I'm not sure which ones were pulling it, but it basically used it every time ANY of thier fleet so much as got it's paint scratched.
I ended up pulling in every ship I had in my fleet, which with full tech development is quite substantial as I also had Large Fleets enabled, and even then after three hours of fighting IN THE SAME SYSTEM, none of thier Capships were so much as damaged, and my fleet basically spent the time just sitting there, wondering if the AI would come out to play eventually.
Think 'Brave Sir Robin' using the mentality of a turtle. I KNOW they have other systems, the asteroid itself had already fallen to me (and most of the infrastructure was already built as well), and yet they still haven't pulled out... but neither will they fight. Can't imagine this should be possible for the Advent to basically play turtle like this, but there it is.