My bikes like to go forward, maybe yours is just showing displeasure at moving backward?
This is the Synchro bike? Despite it shifting smoothly I suspect the chain is simply not perfectly aligned with the sprocket. This happens a lot on friction bikes where the chain is fine under tension pedaling but rolling back with no tension it will slacken.
You may also want to triple check for a stiff link in your chain. A minor stiffie won't be noticeable when pedaling but again with no tension it can cause issues.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk