And it only happens coasting? That's sort of a mystery.
Any freehub related problem would spool chain from the lower to upper loop, and that it doesn't only proves the freehub is working perfectly.
Pedaling or coasting, lower loop tension is purely a function of the idler cage spring tension, and in small sprocket combinations the limit of it's take up capacity. If it only is a problem with small sprockets (both front and rear) than your chain might be too long. If it happens all the time look at the cage tension.
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