Yes the toner can wash out with time it's particularly seen with red fading, that is generally a sign your hair is porous/ damaged, washing too frequently or using too harsh products. You can just use a blonde semi permanent toner with no peroxide to correct that, no damage.
Do not bleach even with a weak solution over the top of the parts that are already bleached, once the melanin pigment has gone and the hair is already blonde the only thing for the peroxide to work on is the structural proteins so you would be doing nothing but damage.
The other way colour can fade is UV exposure, do you wear a hat or close weave scarf religiously all spring and summer? Most deep conditioners hydrate but they don't absorb, protect or repair. Great series on deep conditioning here
THE NATURAL HAVEN: Deep Conditioning : What Ingredients in Hair Conditioner Penetrate Hair?
2a-2c, medium texture, porous/ colour treated. Three years CG, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
CO-wash: Inecto coconut
Treatments: Komaza Matani, coconut oil, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume, Inecto argan
Styler: Umberto Giannini jelly, Boots Essentials gel
Experimenting with: going back to basics