When using either /setblock or /fill, and you're placing a powered redstone item, such as a lit redstone lamp, it will automatically unpower itself after its normal game tick delay. I 100% understand if this is seen as Working as Intended, but consider this. If it's placed as that value, that means said player wants it like that. Perhaps they want a ceiling light for a map that has no way of being otherwise being powered, and they don't want to lag the game with repeating command blocks to make them that way. The way I see it, they should only turn off if they're updated with an active redstone power source.
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Repeating Command Blocks are required if you are doing setblock command, but you can easily reduce lag by setting commandblockoutput to false. The non-command block way of doing it wirelessly (almost) is by placing observer blocks in a way that they detect each other (but this would cause some rare flickers in the lamp).
I have made wireless switches before, and I have a command block that repeatedly sets a redstone block behind the lamp, OR I have a command block test for a signal from the input where the output goes straight to the lamp...
Edit: And yes, this is working as attended (just letting you know)...