I did not notice any particular problem with fleet, and i'm playing the current beta (1.09). Of course i learned while playing that some ships should not be put in the same fleet, and that who is the leader of that fleet decide how the whole fleet will act. Depending on the engagement range and fleet cohesion setting, the fleet may stay in one clump (with lost of ships idling) or spread itself tin attacking multiple targets (making them easier targets for the enemy)...

Edit : I just read what it was all about. I did not notice ships going from one fleet to the other yet, it might have happened in my games though.

Edit 2 : I just noticed that ships did join my fleet even though the auto-join was turned off. In fact, it seem they join when it's off and don't when it's on... Odd.

When you have a mixed fleet selected and give a direct attack order, only the combat ships will obey. The support ships/carriers will continue with their previous order if it was a move/jump. To avoid this, make sure the fleet is stopped in a grav well before telling it to attack, then the support ships will tag along in formation.

As for the OP, I have never heard of fleets randomly trading ships. It's true that if you have 2 fleets in one well, incoming ships will join them somewhat randomly. But they never randomly shift around if you leave them alone.

I tried a post about it last month. The return post I got back were hostile or simply theres nothing wrong. My post is here.

The thing that drove me crazy back when I tried 1.09 was how ships would join the control groups of fleets they joined, but they wouldn't remove themself from the control group if I put them in a different fleet.  So when I went to give an order to a control group, I had ships that had joined that control group detach themselves from whatever they were doing and try to complete the order as well...

I often wouldn't notice this, because the ships would be in a different system by this time, and I would notice groups of ships breaking off what they were doing and flying to join their control groups.  Of course, it took me a bit to figure that out, at first it just looked like ships were randomly doing their own thing.  Pretty annoying.

Could this be what you are seeing?

Not for me, I don't use control groups.

I'd just drive a fleet to some system, have a fight, and when I got ready to leave I'd notice a couple of ships were'nt part of the fleet anymore. This happened more than once.

This was in the previous beta, 1.09, haven't had a chance to try it in 1.095 yet.

I have seen something that sort of similar to this.  This is usually happening late into the game and I never actually see it, but I see the result you can say.  I tell my whole fleet to jump together to the next planet, they are all together and they jump, once they are in the other gravity well, a few of them are shot to the other side...lets say I came into the right side of the well, they came from the same direction but as though from the left side...it happens a few times, hard to notice.

This is a little counter-intuitive. The "auto join fleet" toggle is set for the actual ship to decide if it wants to join the fleet or not. It's not to set the fleet to either accept or not accept new ships. So when you toggled it to off, it meant the selected ships would not automatically join a fleet, but they were already in one. The newly built ships still had it on, so they joined the fleet.

I agree, it would solve a lot of people's complaints if toggling "fleet auto-join" with a fleet selected made that fleet not accept new ships.

I do this this on occasion, usually with a lot of ships jumping. It's usually only a few ships, I think they probably get "bumped" slightly when the ships near them jump, so that throws off their angle a bit and they end up separating. Just my guess, though.