During the height of this heat we've had, my cousin came to my house to make a cake for her daughter since she has no air in her home. Also this was the first cake she's made, so she wanted to do it here so I could help her. Once she finished, she wanted ribbon along the base of the cake. I gave her some that I had that I've used on other cakes. All was good, and she went home. The heat and humidity here has been awful...not sure if that's what played into what happened, but can't think of any thing else. She brought the cake back to her house that she said was hot. She said the ribbon ended up bleeding onto the fondant after sitting in her house for a while. I haven't had this happen with any of the cakes I've done using the very same ribbon.
When she told me this, I cut a piece of the ribbon, and "glued" it to a piece of scrap white fondant, and let it set for over a day. It didn't bleed at all. I did this at my house were the air is on, and we keep the house pretty cool, so I'm wondering could this have been the problem...why her's bleed (in a hot humid house) and mine didn't (in a cool house).
Put that scrap of fondant and ribbon outside your house and I think you'll have your answer.