Thanks for the replies.
We actually got a response back from Microsoft explaining the cause of the issue ...

"On 2025-04-03, control plane requests to Azure Container Apps started failing with error code 500 responses in East US and UK South, preventing control plane operations with ARM, Azure CLI, and the Azure Portal. Container Apps continued to run normally without interruption.

The Azure Container Apps control plane operations were failing internally due to failing authentication requests with the underlying Azure Kubernetes Service (AKS) infrastructure used by the Azure Container Apps data plane. The underlying AKS infrastructure rolled out a change that scaled down the number of replicas of the Guard service, which provides AAD authentication for the environments. The scale down operation caused the Guard service to be unresponsive, which caused authentication requests to fail.

The rollout was reverted by the AKS team, bringing the Guard service back up and restoring authentication capability, allowing the Azure Container Apps control plane to authenticate successfully with the data plane."