buildRapid​Accelerato​rTarget gets stuck during execution in matlab 2020a, but not matlab 2017

I am trying to upgrade a tuning script from Matlab 2017a to Matlab 2020a and I run into an error while running:

load_system('My_Model') %Silently load model 
sltest.harness.open('My_Model','My_Model_Harness');
>>RAE = Simulink.BlockDiagram.buildRapidAcceleratorTarget('My_Model_Harness');

the rapid accelerator build will progress normally until I get the message:

### Successfully updated the model reference simulation target for: My_Model

after this, the rapid accelerator build gets hung and won't exit unless I abort with ctrl+c.

Any idea from anyone what could potentially be causing the issue? Or some way of how to debug? This works fine on R2017a, so I imagine something changed with simulink. My model was created with R2017a, and I am making a copy with R2020a before running it.