The Federal Trade Commission categorizes gold-plating depending on the thickness of the gold coating. Items that are labeled "gold-plated" must have at least a .5 micron thick layer of gold. If the gold coating is .175 microns, the item is considered "gold electroplate" or "gold flashed/washed." Items labeled "heavy gold-plated" must have a layer of gold at least 2.5 microns thick.
I believe the 4mm gold-plated over mystery metal beads were most likely gold flashed/washed because the coating so quickly disappeared.
Research tells me that gold-plated over sterling silver pieces will keep their shine and plating longer than gold-plating over other metals. Most likely if a manufacturer is going to gold-plate over a higher end and expensive metal such as sterling silver, the coating of gold will be thicker or "heavy gold-plated" so as to stand up to years of light use. I think that is why the bracelet made with gold-plated over sterling silver beads looks as good as the day it was made.
Personal opinion: Quality in gold-plating matters. I made myself the same gold-plated over sterling silver bracelet and have been wearing it beside my similar style gold-filled bracelet for several months. Both still look as good as the day they were made. So, any gold-plated beads I purchase in the future will have a sterling silver base. And if for some reason the gold starts to wear off and detracts from the look of the piece made with them, I can take it to a jeweler to have the rest of the gold removed safely and still have a beautiful piece of jewelry to wear. Of course, the surface may need to be smoothed and polished, but that is an easy fix.