It may be that your pedal stroke is sufficiently uneeven that you're engaging and disengagin the freehub: i.e. you surge forward, then effectively coast for a fraction of a second. Thus you have to rengage the pawls of the freehub as you press down on the pedals.

The harder gear in the big ring, pulling more chain per revolution would make that less likely, which explains why it only happens in the small ring.

The chain can't just slip; if it's not adjusted and skipping, it would make a lot of racket and you'd know it.

It's possible the pawls in the free hub are sticking and it's not engaging and the freehub is slipping.

It's also possible the cassette is catching on the spokes, and interfering with the freehub working properly.