in sativa dominant strains mostly, its where you get some new white hairs poking out the tips and it looks like new growth, but skinny growth at the ends going into pairs of calyxs, i have seen some high quality bud like that and there is nothing wrong with it, sometime heat can caue it, humidity, but mostly strain relevant. i had it on just one large cola on my first grow, it was closest to the light, every other head matured quickly, but the one that foxtailed just wanted to keep growing, and growing and get soo damn fat it was ridiculous, it wasnt imature, just didnt want to finish, maybe a different case with you, i think yours is purely strain driven, and its growing some skinnyish tips with large calyxs, the buds themselves however are filling out beautifully and are 100% healthy to say the least
don't worry bout what i say, enjoy growing that fine strain! and keep those bits for yourself, trust me
reminds me of shiva shanti i once smoked, real fox taily, but man did it stink, and get you high as a kite!