I noticed that my culture was not spreading correctly after I took over planets. I eliminated one team and their culture was still connecting their planets that I owned and each planets description said they were still the dominant culture.
I then eliminated everyone and owned all planets. The other players culture was still dominant on their planets until I built broadcast centers on each planet that had a phase lane a different color. However, my culture only spread to the next planet and no further.
I then noticed when a planet broke a phase lane connection due to planet orbits the phase lane would not turn red (my color) when making the new phase lane connections to the new planets. I had 20 or so planets out of 40 with max broadcast centers so I should not have ever seen a phase line that wasn't red (my color).