Comparator next to the dispensers & droppers not powered properly by redstone signal

Put the summary of the bug you're having here

What I expected to happen was...:
Dispensers & Droppers interact with redstone power like other non-transparent block.

What actually happened was...:
Comparators are not activated next to the weak powered dispensers & droppers.
Also it happens when directly strong powered by underneath redstone torch.
Steps to Reproduce:
1. Place dispenser or dropper
2. Place comparator next to them
3. Weak power a block