Next month: January 2020
It turns out that it was a basic error on my part - I'd defined several arrays to the size of the number of main Lotto draws and then also used them for Thunderball results. This was fine until the number of Thunderball draws exceeded the number of main Lotto draws, which happened on Thunderball draw number 2441.
The fix was quite simple - define the array size to be whichever was the larger of the number of Thunderball vs. main Lotto draws (which will be Thunderball from now on).
Previous month: November 2019