I had a ticket created a while ago about this behavior: #AJZ-662-10426, with a recent save game.
In this game, I'm playing at gifted. I don't know if it's WAD because of the low difficulty level (I haven't played in a while, like 2 DLCs ago at least...), but as soon as you have drones defending a planet, like an orbital platform that creates renewable drones, the AI will always stick to attacking these drones no matter what is beside it.
You have a shipyard producing a brand new dreadnought turn after turn? No problem, the AI will attack the drones. You have a huge fleet nearby that albeit its size would likely or surely be defeated by that AI fleet? No problem, the AI will attack the drones.
The only exception to the rule are fleets containing transports. So if the AI is attacking a planet with drones and you are attempting to invade a planet next to it, or if your transport fleet happens to come by that attacking AI fleet, than it will react. Otherwise, no attack, no invasion.
In this game, the AI could have wiped the floor with me given how badly I played early on. Even now, most of the AI fleets are better armed than mine and could destroy my starbases, my fleets, invade my undefended planets. But instead, it chooses to fortify its planets with troops waiting for me to invade.
My next game I'll crank the difficulty a couple notches higher to see if the AI still repeat that behavior. That bug has been there for a long while... It'd be nice if it were fixed for the next patch