They only split and merge in lower tiers due to a lot of movement in and out of each league. Once you reach platinum, especially platinum I, you'll rarely see any merging or splitting. Try taking advantage of a contest where you have to get FP and Coliseum wins to advance to the next tier. That was how I went from Plat II to Plat I in the fight pit. A lot of people were lowering their defenses and attacking only push-over teams, so I put my best team forward, snagged 2nd place, and held on for two days.