YOu should only use 50/50 once. Distilled water thereafter. PG does not evaporate, just the water. Keep adding 50/50 and you will keep changing the ratio, which will alter the humidity target.
A full humidor that doesn't leak should not require much in the way of adding water, and virtually none when the heater isn't sucking all the moisture out of the air.I would throw that moldy credo out, get a new little one to set in there, and start over.