Integrator block integrating "the wrong direction"

In a simulink realtime model (external mode) I have an integrator that seems to integrate wrongly. A negative input signal of around -10.8 (constant except for noise) is integrated to a positive slope of about 1.5 and only when the signal drops to -14.8, the slope becomes negative (ca. - 2.5). I use ode4. When I try to reproduce the issue in a second model (save the data to the workspace, use it as input in the second model), normal mode with ode45, the integrator behaves as expected: First negative slope of about -11, then a steeper negative slope of about -15.

What am I doing wrong in the external mode model?