A deactivated comparator does not produce any light, as expected. (Picture 1)
However, an activated comparator does not glow either. (Picture 2)
Instead there is an unused activated comparator (ID 150), which does give off light. (Picture 3)
If this unused comparator is depowered and activated again, it turns into a regular not-glowing comparator. (Picture 4)
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Duplicate of MC-8784.
I blame the search tool for that.
I hope this bug hasn't been reported yet, but at least I couldn't find it when searching for "comparator".