I believe there is a problem with modifying tunable parameters when using parsim in 2018a. I have been having difficulty getting rapid accelerator parameters to update from one simulation to the next. After struggling I decided to work through an example in detail. When following the example "Rapid Accelerator Simulations Using Parsim" (found in the help) in 2018a I noticed that were was no variation in the results even for the example. The first image below is from the help of 2018a. I then tried the same example on 2017b and got the results I was looking for. The second image is from the help of 2017b. Notice the variation in this example. Again, these are the result plots published in the matlab help and I have verified that I get the same results when running on 2018a and 2017b. There appears to be something wrong with 2018a that is not allowing the tunable parameters to be updated.
There is another example that behaves the same way, "Parallel Simulations Using Parsim: Parameter Sweep in Rapid Accelerator Mode" available in the 2017b and 2018a help.
Not sure if this is related but I notice that "SimulinkGlobal (Auto)" is not an available option in the Storage class drop down in the Simulink model's "Model Parameter Configuration" dialog.