Hi Peter Boulton:
Thanks for reaching out to Microsoft Q&A.
I thought setting a ceiling amount would prevent this - surely it should?
You will be able to take action when reaching a certain threshold cost through actions which can be in the form of Azure functions, webhook, logic apps, where you get the privilege of stopping the services that incur cost and you want them to be stopped. When creating the budgets, it will ask you for the actions to be taken as below.
I logged in again and initiated a delete of the SQL Managed Instance, which is stuck on stage 3, 8 hours later
did you see the same even after refreshing the webpage or after logout and login? if it is so, please raise a support ticket with the azure team. Some operations might get stuck in between which the backend team has visibility to analyze and resolve.
https://azure.microsoft.com/en-us/support/create-ticket/
Please 'Upvote'(Thumbs-up) and 'Accept' as answer if the reply was helpful. This will be benefitting other community members who face the same issue.