How could this have happened to gold-filled jewellery?

Hi. I purchased a ring which I was told was 'gold filled' about 3-4 months back online.Not being very sure about the difference between gold-filled and gold plated, I read up about it thoroughly and also questioned the supplier about durability, characteristics, care etc.. I know the gold layer is heat bonded to a base metal and the thinkness of the gold (this is yellow gold 14 kt) needs to be 1/20 of the whole piece and the like

Anyway, I was made to understand that the colour will not tarnish or fade if the piece is looked after carefully and that it will retain it's colour for 20 -30 years.Now of course I wasn't planning on keeping the ring for so long but I expected it to last at least 2-3 years. But after just 4 months the colour is fading. Note- fading- not peeling or flaking like plated metal- just fading. the yellow is gradually fading to a silver-like colour.

I wear the ring everyday but remove it before a shower or housework etc...

FYI- there is a possibility that it was dropped in some kind of chemical(possibly bleach!!!!) to test whether it was genuine gold (DONT ask, long story!) not by me of course.

Anyway what I need is some explanation for the colour fading. I mean, even with daily wear, I was pretty careful with it so what's the deal? Also, I was so impressed with the range that I bought several others for myself as well as gifts and it would be a huge embarrassment if they turned colour within such a short time. Please help.

M