Yes, as you say, strange :). It looks like a bug to me.
My goal is : mixing a shader of bottom and leaks dropping texture with a other shader using the Alpha channels - where the amout is controlled by a Float Value node (Colorramp in first place, my mistake) - as mix factor. Pretty simple in fact :).
And I would like to use only 2 nodes as there is no reason that the Alpha channel being altered by the fact that Color output is also used.
I used this for this render : Welcome