The output of a comparator, which is drawing a signal from a block/minecart with inventory or command block through a block, can only be overridden by a redstone signal strength of 15.
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duplicates 1
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"With inventory" is not sufficient information to determine if this is a bug. How full is the inventory? How many times did the command block's command succeed?
It doesn't matter. The point is that whatever signal (or lack thereof) is being passed through the comparator can only be changed if the block behind it is fully powered.

Is this still a concern in the current Minecraft version 1.8.1 Prerelease 3 / Launcher version 1.5.3 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Forward resolving to MC-64394, in favor of the information provided there.
Confirmed in 13w16a.