I've noticed that using AppCompat themes, default toolbar icons get tinted by the attribute
<style name="MyTheme" parent="Theme.AppCompat">
I see this question is getting some views so I'm going to post an answer for those who don't read the comments.
My conjectures in the question were all wrong and it is not a matter of alpha channels, at least not externally. The fact is simply that, quoting @alanv ,
AppCompat only tints its own icons. For now, you will need to manually tint any icons that you're providing separately from AppCompat.
This might change in the future but also might not. From this answer you can also see the list of icons (they all belong to the internal resource folder of appcompat, so you can't change them) that are automatically tinted and with which color.
Personally I use a
colorControlNormal which is black or white (or similar shades), and import the icons with that particular color. Colored icons on a colored background look a little bad. However, another solution I found pleasant is this class on github. You just call
MenuColorizer.colorMenu() when you create the menu.