Changing input background to black and foreground to white does nothing to a placeholder - it stays black.
Seems like it should be also changed with foreground color.
That’s not a bug. Unfortunately CSS doesn’t work like that.
Ways to change placeholder color have been asked many times on the forum so you can search for them. The only way to change the placeholder colour appears to be CSS
Yes, it can be replaced in CSS, same as you have %color% variable for other elements. It is more complicated because of rgba but… I still call this a bug 
Or ok, not a bug but unfinished feature on that component.