Candles will start emitting light before the particles appear, and it will appear as if an unlit candle is producing light before the flame particles show. To recreate, make a room with no light inside and place a candle, then light it.
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We're tracking this issue in MC-203558, so this ticket is being resolved and linked as a duplicate.
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Hey there!
I don't believe this is a bug. If you think about it in a real life scenario, you light a candle and the flame grows, until it eventually maintains the same size. That process of the flame reaching its maximum potential its well represented in Minecraft so I feel this isn't an issue.