Sorry about that! I think I may have confused two related forum questions with each other. There are definitely some unexpected results with the multimode components co-simulating with amplifiers, mostly I think they can be fixed for now using the Convert to Sampled Signals and Signal Type Selector components. Although it should not be this complicated and the OptiSystem team is working to make these simulations run smoother.
That being said I tried with some amplifier components we have and got results I was expecting. Which exact fiber amplifier component are you working with (Er Doped Multimode Fiber)? Attaching your project file might be the best way for me to find the problem.
Thanks