No it's not a major fault. Not all, but alot of brindles have white patches. Where you describe the white patches and w/ her age her white patches may fade as she matures. I have a brindle girl and she had white on her feet and a white toenail, but now as a 5 y/o they are gone.
my boy is brindle and also has a white patch on his right hind paw, thanx for asking this question, his pad on his toe is also white.
I have a 3 1/2 yr old Fawn Mastiff and he has one half white toenail but no white fur patches. I've heard from other people - not having had Mastiffs - that he could be a cross breed. It sounds like it might be normal from the others that have posted. Glad I found this sit and thanks for asking the question!