OP Did not say they cancelled both, just that they cancelled. I have no problem with automatic cancellations, particularly at high demand periods like
Free Dining, holidays, etc. Disney has a responsibility to give guests, esp. DP guests a chance to use their credits. This is just one way they can help manage the process. The OP had two ADRs during the dinner hours on the same night, dinner seems to be any ADR past 4pm. Disney's position is to allow other diners a chance at that second dinner reservation. Obviously a way around this is to put the second ressie in someone else's name and phone number. Probably not in the spirit of the policy, but follows the letter.
Hopefully Kona was cancelled. I would suggest OP show up at Kona close to closing stating they just wanted to order dessert. If there are servers and seats open they'd probably take you as dessert and coffee are high margin products. Otherwise I hope you find something absolutely wonderful on the CG dessert menu.
Just keep in mind if you're on the other end of this, really want to eat and maybe really want to eat at Kona, you might be ecstatic to get an ADR, even at 9pm.
We have been fortunate in getting ADRs at every place we wanted and even adjusted times as our plans changed. Coral Reef was the toughest to get and I'm not sure if we received some auto cancellation pixie dust or what, but I am grateful that we'll get to have that lunch experience during our trip. Since getting that ADR I've called to add or verify what was on our list and several times the CMs said, "don't change that one", never our intent, "because there is nothing available".