anonymous user Thank you for contacting us!
AFAIK, there is no issue with the REST API or with the billing. This is expected in most cases. Because there must have been some changes happened or took place each day. For example: A resource must have been deleted or in case of Azure backup if retention was set to delete data after few days, in that case the storage cost would reduce. This is just an example, there could be many other factors that might bring fluctuations in billing. The best I would recommend, is to contact Azure billing support team by raising a support request. This is a free support, wherein you will be in direct contact with the support professional who would help you better understand the billing for your subscription.
Here is the link - https://learn.microsoft.com/en-us/azure/azure-portal/supportability/how-to-create-azure-support-request to create support case.
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