Janssena:
What you can see above is that A[1] is filled with the dose from the callback, causing dA[1] to be negative. This time however u[1] remains 0., causing the result to become negative. After some time the concentration in A[2] is large enough to actually start filling A[1], so A[1] becomes postive again.
Is this a expected issue when using interpolation? I of course can manage with using saveat, and setting save_positions = true when I need to use interpolation, but I would expect save_positions to only save the positions to the sol.t, and not affect the solution in other ways, as seems to be the case.
Yes, it’s expected. If you don’t save the discontinuity points, how could you ever interpolate correctly? You need to save each of the points and two points at a jump discontinuity in order to do it.