I have lots of questions.
Was this same display working correctly in the past?
(I couldn't tell from your initial post)
Silly question but do your power rails on your breadboard
run all the way across?
Some breadboards don't connect between the individual sections.
Was the display ever miswired even temporarily?
Specifically on the contrast pins (pins 3 and 18)?
(hooking pin 18 even momentarily to gnd or vcc can fry the onboard Vee negative power supply)
What value pot are you using?
Describe more of what you mean by:
The contrast pot is wired correctly and works as expected.
The reset seems to be happening properly now using the handy feature in the new library. At least I assume it's working since the upload is working without error and the display clears and cycles during restart.
So for clarity, there are 3 wires on the pot, 1 end is to 5v, 1 end is to Vee (glcd pin 18), and the middle
wire is going to Vo (glcd pin 3)?
Note: for this next part don't turn the pot all the way, just enough to get the result.
If the contrast pot is working as expected, when you turn it one way all
pixels disappear and the other way they all turn black no matter what the glcd controller
and glcd memory are doing.
Is this happening?
Are you now using the software reset using pin 19?
What do you mean by "display clears and cycles during restart"?
Can you measure a few voltages?
(be very careful not so short out the adjacent pins - its easy to accidentally slip off a glcd pin)
From gnd to VCC as measured at the GLCD. (between glcd pins 2 and 1)
Now remove the wires on glcd signal Vo (pin 3) and Vee (pin 18)
Measure the voltage between gnd and the Vee output (pin 18).
(It should be negative 6 to 10 volts)
Now hookup the contrast pot again.
Measure the voltage from gnd to Vo (pin 3)
This again will be negative when you can see
the pixels but will vary depending on the contrast setting.
I just measured 2 different displays and 1 was -2.5v (green/yellow background with black pixels)
and the other was -6.5v (blue background with white pixels)