I don't have this issue right now, so I assume the bug has been fixed. However, there is another bug regarding String Join action right now. It seems like the last value is not added to the resulting String.
If I run String Join action on String Array of [100,200,300,400], the final resulting string is 100,200,300.
I tried various sizes, and the last element of the array is always missing.
I am using the latest Unity 5.4b20 and Playmaker 1.8.1