I broke a black stained glass pane with a silk touch pickaxe and it dropped a white stained glass pane

What I expected to happen was...:
when i break a black glass pane it should drop a black glass pane

What actually happened was...:
it dropped a white stained glass pane

Steps to Reproduce:
1. place a black glass pane
2. break it with a silk touch pickaxe
3. see what drop