I’m a little puzzled by the whole “walking away from you” and “Master” bits in the trailer, since they didn’t really have a relationship other than fellow insurgents in Rebels.

Remember that Rebels ended before the Battle of Yavin. There were about 5 years between the main story finale and the epilogue, which would have been plenty of time for Ahsoka to spend some time training and then subsequently abandoning Sabine ahead of their reunion shown in the epilogue.