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.
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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.