Since I make my own tissue and I use the minimum amount of pigment, my highlights will actually be made of a thicker layer of gelatin than using a tissue with a much higher pigment amount. This is what could be protecting my highlights from being 'eaten out'.

I use 4 grams of lampblack watercolor paint (from tubes) to 750 ml water, 90 grams gelatin (food grade) and 60 grams sugar. Different brands of watercolor will contain different amounts of lampblack...so one needs to standardize on one brand if one goes that route.