The AI will automate (and thus override) fleet postures if both the fleet is automated and fleet formation is set to automated. Check the empire's policy tab for it. Postures won't work if the fleet itself isn't automated.
I was tearing my hair out over this as well until I figured it out. When you turn it off, the AI won't create fleets for you anymore so most of your ships, if they're still automated, will fan out and take on escort/patrol duties or just putz about in orbit where they were constructed.
But that isn't a bad thing. It's not that difficult to set up your own fleets even with large empires since the screens load so much faster. When you build a set of ships, do so from the Construction Yard section of the ship screen (from the tabs at the top of the screen), then switch to the military ship section and check for the ships you queued, their names should be in orange. Select them, then assign them to a fleet. You can then manage your fleets from the fleets screen.
To keep tracks of fleet, I suggest using a naming system. In my current game, I use the fleet designation to remind me of what areas the ships should operated in (1st is Core, 2nd is Middle colonies, 3rd is Outer). Then the actual fleets are task forces, followed by their role in paranthesis. It also looks really cool from the galaxy map view when they're flying from mission to mission.
1st Fleet, BTF-1 (Core Defense) [<--12 Cruisers, 4 Carriers, 2 artifact capital ships. Home system defense]
1st Fleet, BTF-2 (Core Defense) [<--1 Cruiser, 3 destroyers. I cleared out the most dangerous pirates some time ago, so they deal with the dregs. Also, my destroyers are size 400 :P]
1st Fleet, BTF-3 (Core Defense) [<--1 Cruiser, 3 destroyers]
1st Fleet, RTF-1 (Rapid Response) [<--2 Cruiser, 1 Troop Transport for responding to raids]
1st Fleet, RTF-2 (Rapid Response) [Ditto]
Hope that helps!