After some step-by-step debugging, I think I may have found the bug that caused this issue.

Look in calendar.js and look at line 595 (at least it was for me) and look where it says...

var other_month = !(cal.dateClicked = !el.otherMonth);
if (!other_month && !cal.currentDateEl){

And change it to...

var other_month = (cal.dateClicked = !el.otherMonth);

By removing the ! at !(cal.dateClicked...); I was able to have it work correctly for me, and everything else still works correctly.

Now, I make no claim to know why this works. All I did was trace where the function fell apart at & noticed the exclamation point and removed it to see what would happen.

I have only tested this in the case where I had a flat calendar, no multiple dates, and had special dates with the rest disabled. Results may be different if you have a different setup.

Perhaps when I have more time I will see about disecting it some more and see if I can find out exactly why it does this.