Unfortunately there is no rhyme or reason to the dining plan. There has been NO increase on the adult plan since Jan 2018. The kid's plans went up a few dollars each starting Jan 2019. I would expect an increase on the adult plan before the end of the year, but even if they do, there's no guarantee they won't go up again before 2020.
However, the cost of the plan at the time you book is the cost you'll pay even if there is an increase. So if you think you may want the plan, book it as soon as packages come out for 2020. You can always remove it later if you decide you don't want it.