I’ve never heard of changes in pigmentation because of drool rash. Drool rash on itself is quite innocent and will disappear when the child gets older. I always kept the face dry by dabbing with a soft face cloth and apply a barrier creme like Bepanthen or Vaseline. Coconut oil should be fine too.
But when you have concerns about the loss of pigmentation I would consult the GP too.

This isn’t something I’ve heard of before so I would consult my doctor.

She will probably grow out of it

Medical advice would probably be best

A GP can refer you to a specialist.

I would seek specialist advice and support.

I’ve never seen this before, what has the GP said?

Once pigmentation is lost, I doubt it will come back. I would suggest a visit to a skin specialist, see what they say

have a chat with your GP they will be able to give you the best advise.

I’d see a skin specialist I know some products do strip pigment but it’s usually steroids etc.

I don’t think there’s anything you can do to bring the pigment back. Perhaps just a thing that will correct it’s self over time and the skin sheds and renews itself.

I don’t know that it would be the same but my sister lost pigmentation on her thigh from an ongoing skin condition (doctors couldn’t come up with an answer to what it was). She had it for over a year and the structure as well as the colour were compromised. She was told that it was a form of scarring and may lessen over time (she has quite an olive complexion). The structure took some time to smooth out (over a year) but she had resigned herself to the loss of pigmentation….but after around 3 years it’s like she never had it. Hopefully this is something that will eventually resolve.