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.
{F5704162}
(the hour was 00:19 in my digital clock when I made the screenshot)