There are a few distinct shades of pink used in G1s; all of them are more prone to fading than any other colour because pinks and red pigments are the most volatile. However, there is of course the one shade [as per Posey, Wind Whistler] that fades if you mention sunlight.
Baby Bow Tie [and adult Bow Tie] do not have that colour of pink - its a deeper pink shade. So while yes, with exposure to the sun, it will fade it will not fade at anywhere near the speed of the fading pink shade and it will take much more exposure for longer periods to see a difference.
In the same way pink hair is prone to fading, so is the pink pigment in paints - so its surprisingly common for some lighter pink symbols to bleach out. A lighter pink symbol will also fade out much quicker than deeper colours.
So its a balance of time. Clearly poor little Bow Tie has been exposed to the sun or UV long enough to fade her symbols [which can be a surprisingly short period of time] but not quite long enough to fade her hair [which for that shade is quite long].
I have had a couple with white symbols over the years, I think she is enchanting that way.