Why does the Simulink point-mass high-precission orbit propagator increase semi-major axis over time?

I am using the "Orbit Propagator High precision" built-in Simulink block.

I have set the gravity model to point-mass, and the input and output frames as ICRF (as they were).

I do not select a specific solver. I fix a step-size, but let Simulink automatically select a solver.

When I run a simulation, and convert the X_icrf values to orbital elements, I see that the semi-major axis continually increases over time.

Shouldn't it remain constant for this setup?

Could you clarify why the "point-mass" model is outputing such behaviour?