Hello again. Rose, I believe Miss Rose means FAT from the EGG YOLK. As you know, sometimes when separating whites from the shell the yolk breaks & a slight amount of the yolk gets imbedded in the whites. This small amount of fat can play havoc with the royal icing. I hope this explanation will put your mind at ease.
Good day my friend.
The fat from the butter in the buttercream is really the culprit! Over time, the royal icing will begin to “melt” and disintegrate. As for how long it takes before it happens, your mileage may vary based on where the cake is stored (high humidity or low humidity refrigerator) and how far in advance you pipe the royal icing onto the buttercream.