All the one's I have here that aren't clipped are also losing a fair bit of coat Susanna, a wine coloured carpet in the lounge is proof of that

but I've never had them lose hair off their tails the way you've described Rhians hair loss, very odd

I doubt it would be dietary because you say the rest of her coat isn't patchy or dull.
This might give you a few pointers to hair loss symptoms, looking at the list I wouldn't think there are many relevant to Rhians tail though
'Callus' or persistent pressure (mostly in giant dogs)
'Cushing's Disease' or high level of corticosteroids
'Demodectic Mange' or infection caused by the Demodex mite
'Diabetes Mellitus' or diabetic conditions in dogs
'Folliculitis' or infection in the hair follicles
'Sarcoptic Mange' or infection caused by the Sarcoptes mite
Tail Gland Hyperplasia' or enlargement of the sebaceous gland of the tail
Allergic reactions to various substance like metals, chemicals, mites, pollens or certain food
Certain growth hormones lead to hair loss. They are witnessed during puberty and mostly in puppies
Excessive estrogen in the body or 'Hyperestrogenism'
Excessive stress (observed during pregnancy and nursing)
Fungal infections like ringworm
Insufficient hormone production by pituitary gland (Pituitary Dwarfisim)
Low estrogen level or 'Hypoestrogenism'
Low thyroid hormone production or 'Hypothyroidism'
It could just be a shedding process she goes through but I can't say I've come across it before, not tail specific anyway, sorry, maybe someone else can offer advice
Sandy