A conditional command block with the command "/testfor @e[type=Skeleton,r=50]" has an output of "[13:02:35] Found Skeleton", but a comparator placed facing away from it does not output a redstone signal. This is tested and does not work with any type of conditional command block.
The same command does work when the command block is not conditional.
Comments 3
If a conditional command block is powered but the preceding command block didn't succeed, the conditional block won't run but will also deactivate all comparators facing away from it. This is intended behavior though.
The success message inside of the conditional command block may come from an earlier execution and just hasn't be overridden as the command block didn't execute again since then.
Does the time of the output message change when you power the block?
Could not reproduce; outputs comparator signal as expected. Please provide an image of your setup. Make sure that the conditional command block is succeeding (has a successful command block behind it).