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Why does an appended "abs"-block change earlier results?

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I've got a Simulink model, simply a filtered step given as reference onto a control loop with PI control. There are Delays both in the plant as in the measurement feedback. As the simulation results oscillate heavily (wrong) I take ode15s as Solver. The result is good.
Now I add an "abs"-block to the system output, i.e. not inside the loop. The result BEFORE this block still looks good, but different from before. How can this happen, which one is trustworthy?
Swarooph on 26 Oct 2016
This seems like a solver related issue. Adding an Abs block maybe triggering extra zero crossing events which may affect how the model is solved as a whole compared to just that one block. Without looking at the actual model, this is just a guess of course. If you are new to the concept of zero crossing, take a look at this example and draw parallels with your application.

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