Once followed the steps on the guide to activate Oauth for RabbitMQ: https://techdocs.broadcom.com/us/en/vmware-tanzu/data-solutions/tanzu-rabbitmq-on-cloud-foundry/2-4/tanzu-rabbitmq-cloud-foundry/enable-oauth-for-rabbitmq.html
You will notice that the service instances that are in the platform before activating Oauth won't be accessible and will fail to log into it.
RabbitMQ on any version after activating Oauth it won't be possible to log into old Service instances management UI.
This is done by the design of the service broker which communicate directly with TAS UAA, It has information about newly created service instances but not the old one's
There isn't a workaround to make service instances active before OAuth was activated to work with UAA.
Customers should be aware that activating Oauth on a running environment will cause previous service instances to be inaccessible.