Hi Diego,
It is normal to be charged for managed disk for ML compute cluster. The charges should only be for the amount hours that the cluster existed.
Please create a new billing support request using my instructions below. There is no charge for billing support.
Use this link to start the process:
https://portal.azure.com/#view/Microsoft_Azure_CostManagement/Menu/~/support/openedBy/AzurePortal
Fill out Summary, Problem type, Problem subtype, click Next. There will be a delay, then some recommended solutions will be shown. Do not click on any of the recommended options shown.
In the upper left corner, click Return to support request
Click Next to move to Additional details tab, fill out required information, email/phone, etc., click Next, review details and submit case. After submitting you should receive an automated email with a few minutes and a support engineer should contact you within 1 business day.
Please click Accept Answer and upvote if the above was useful.
Thanks.
-TP