Bhagavad Gita, Chapter 9, Verse 21

Having enjoyed the vast Swarga-world, they enter the mortal world, on the exhaustion of their merit: thus, abiding by the injunctions of the three Vedas, desiring desires, they constantly come and go.

Even a real, considerable reward runs out eventually, and what follows is a real return to where things started.

This names the actual mechanism honestly, merit spent, no exception, no permanent escape through this particular route. The cycle described here is real and repeating, not a one-time trip.