Based on my talks with breeders of Onyx clowns, I think it goes beyond the clowns and anemone combination. For example, I believe that environmental conditions are a factor. I spoke to a reputable breeder (known for their jet black Onyx clowns with a high percentage of black dorsals) who mentioned they blast their clowns with bright light, which they believe helps to develop the deep black.
I was told by an anemone expert that all of her clowns shifted colors in all anemones except Mertensii (and I've never seen these for sale).
I also believe that food fed to the clowns can make a difference.
Long story short, not every anemone combined with every Onyx will cause color shifting, but based on what I've seen, I do believe that there is a greater possibility of blacks fading with Onyx are combined with BTAs.