(Spoiler: Why adiabatic expansion happens in Carnot cycle? doesn't really answer the question for me.)

In the Carno cycle, the open system is first brought into contact with the warm reservoir, which then expands isothermally.

In the second step, the system is separated from the reservoir and is now thermally insulated, resulting in another small adiabatic expansion.

I ask myself, why is a little internal energy converted into volume work after the separation? Isn't the equilibrium already reached after the isothermal expansion? (obviously not, but why?).