DP pin is not initialized by default

[zlietapki]

name.setSegment(DP, HIGH); just do nothing
because error in line
for(uint8_t segment = 0; segment < 7; segment++) pinMode(segmentPin[segment], OUTPUT);
this line should be changed to
for(uint8_t segment = 0; segment < 8; segment++) pinMode(segmentPin[segment], OUTPUT);

as a temp solution you should use
void setup() {
pinMode(<dp_pin>, OUTPUT);
}