Hours, minutes and seconds are in hexadecimal system. I don't know if it was intended or just temporarily ported to Plasma5 that way but it's not an expected behavior from my perspective.
Basically a column gets cleared if the column on the right of it is full. Which means that when a column reaches 16 it becomes 0 again and the column on the left is incremented by 1. And it should happen for 10, not 16.
Here's a screenshot of 2 widgets. On the left there's my fixed Binary Clock widget with decimal system and on the right there's an old widget displaying time in hexadecimal form. Screenshot was taken when my Digital Clock showed: 00:19.
{F5704162}