If you didn't change anything like chain length then I suspect you've somehow gotten the freehub (or freewheel) stickier.
It's only slight, so you only feel it when the chain is slightly slack, otherwise the RD cage is enough to resist the force. You can test for this by shifting to the small/small combination with the bike off the ground apply your brake to stop the rear wheel. Then watch the upper loop as you give the wheel a push to spin it forward. I suspect that you'll see the upper loop immediately sag somewhat.
The effect is mos pronounced in the smallest rear sprocket because the chain has less leverage to hold the cassette back.
Take the wheel out and feel to see if the freehub feels a all sticky, and if so, you'll want to clean and lube it.
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