[Soulfisher]

I was having the same problem in my simultaneous game under se4 Version 1.78. I patched the game to Version 1.84 and continued playing the saved game. Now the AI (in this case a "human" player set to use all it's ministers -- this is a "team" battle of human (me and my allies) verses the computer team) has accepted a trade of specific planets for one planet of theirs. When they accepted the trade, my offered planets were now under their control, and his planet went to my control.