Hi jones,
Not sure, if you already found a solution but I think the problem might be, that the RAP is not able to get an inner IP address from the cluster as the config for this is different than in earlier versions of AOS.
Here is a link where I describe the setup with a cluster. Maybe this will help you:
https://www.flomain.de/2019/05/basic-rap-setup-with-arubaos-8/
The first part of the Cluster paragraph is important. The setup of the "Controller Cluster RAP Pool" in the MM config mode.