Like I said, I’m sure I saw through April, so yes, something is odd.
My guess is that the December 2016 data is a default placeholder used when they are updating the current registration figures and have yanked down the last monthly data set but have not yet posted the new one. For some reason, the May figures were seen at the last minute as not yet ready so what should have been a smooth process was interrupted. Notably, some local county Supervisors of Election have posted registration data compiled as recently as June 1, which is functionally an end of May data set.