you lose AE for giving up provinces or releasing vassels. it was possible to have say burgundy as vassel, conquer some french alnds, sell them to burgundy, then lose a war on purpose when you have large AE and give those lands back for a lot of AE reduction, then wage war on france again and force them to return the provinces you just gave up by forcing them to give back burgundies cores, resulting in no AE< resulting in a situation where you can have very little AE by lowering it with a few 100 every 10 years without losing any actual land.that exploit still works, but now you'd have to conquer some lands yourself and give them away and take them all the time, which makes more sense and is much more annoying for the player with the fluctuating forcelimits ect. and IIRC loweiing AE is now also done on local level so you no longer get global reduction for it, forcing you to do that on EVERY front.