simple model has different results with timestepping branch

I have a simple test model (ABtoC in the GUI, with initial concentrations set to spatially uniform)

A and B react to form C. So C should grow linearly with time from zero and be spatially invariant (see first image)

With the timestepping branch (second image) the results don't match the analytic solution any more.

Simulation using master dune-copasi branch:

Same thing using timestepping branch: