Hi Toby,
The discrepancy you are facing now as it had not happened earlier can sometimes likely tied to changes in Azure's infrastructure or operational policies and you don't want to follow the workaround for a resolution by trying to stop and start, as the process normally launches automatically using a Function App to periodically start container instances.
If possible, check once the availability and quota limits of Azure Container Instances compute, memory, and storage resources in Azure regions and by target operating system. https://learn.microsoft.com/en-us/azure/container-instances/container-instances-resource-and-quota-limits
Please check the similar issue workaround here for more detailed information as here it was suggested it was an issue in Microsoft. https://stackoverflow.com/questions/77187248/azure-container-instance-is-still-transitioning-please-retry-later
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