Mixing a banana with homemade sunflower butter turned into a horrible, leathery substance. Why?
I suspect the textural change you experienced is analogous to the seizing of chocolate (which is also a ground, fatty seed--in that aspect similar to a sunflower seed).

Sunflower seeds are fairly dry.  The consist mostly of fat, and starches, and protein.  When you add a small amount of water (from the mashed up banana), the starches in the sunflower hydrate and absorb the water, making it thick and giving the textural transformation you experienced.

If this hypothesis is correct, adding enough additional water to fully dissolve and suspend the starches will give you a smooth fluid, or paste again.

As to the taste perception, that I cannot comment on.