Hallo Gabriel,

soultion is quite easy. If the change requires a 0-to-1 transition in Port1 an addiditional pullup is turned on during S1P1 and S1P2 of the cycle in which transition occurs. The extra pullup can source 100 times the current that the normal pullup can. So when you try to drive your rather low ohmic load (your resistors) only extra pullup can guarantee correct level. Afterwards, only very weak pullup is driving your load. But load is to heavy, weak pullup fails and voltage at port pin breaks down. That's the reason why you only see dots.
Solution: Insert 74HC04 inverters. They are much stronger and can drive your resistor ladder.
You will not succeed with additional resistor pullups at Port1 pins, only, because load would be much to high.

Bye,
Kai