I just dealt with a points allocation issue, which I still have to deal with further in a couple months due to holding limitations. But I was surprised to learn how points were allocated, but luckily Disney was able to help me re-allocate them appropriately. To your question.
As in my case, any new reservation will use the points that are currently available. So your June reservation will use the 2018 and 2019 points originally booked and the 59 remaining borrowed points will be used in the second reservation. If/When the December booking is canceled then the 2019 borrow points will be added to your available points in 2018. You would have to call DVC before your June trip to have these points re-allocated to draw from all the 2019 borrowed points (I just did this and it was pretty easy), otherwise the June trip will use the originally used 2018 and 2019 points leaving you in a use, rent, or lose situation with the 2019 points borrowed for the December reservation. With that all in mind, if you go on the June trip and dont cancel the December trip until after your trip, you will be stuck with the borrowed 2019 points to be used in 2018.
Also, as a reminder, if you do bank your 2018 points they will have to be used in 2019, whereas if you dont borrow the remaining 2019 points they could be banked to 2020 if desired.
I hope this helps and with the caveat that this is my understanding of the system and not guaranteed to be 100% accurate