When you place redstone wire on a block of glowstone and connect it downwards to a glowstone block next under the other, then it results in the lower redstone wire note being powered.
The same problem occurs when using half slabs.
Linked issues
Attachments
Comments
CubeTheThird
MinecraftWiki has some information about Slab Diodes, which ensure circuit insulation by preventing downward connection, in the Redstone article.
You should read through that.
This is not a bug, but a feature. You will notice that the redstone can be powered upward, but not downward on these blocks. This can be useful in creating certain circuits.