By 'last digit' I assume you mean the one connected to D5 on the schematic.
The PIC has run out of pins to drive 5 digits so the way it works is to drive one of four or none at all, the none at all condition being detected by T1 which forms a crude NAND gate. I suspect the problem lies in the slow respose of the gate circuit, try replacing R10 with a signal diode in the same way as D1, D2 and D3 the connecting a 2.2K resistor between the base of T1 and VSUPP.
Brian.