Hi,
There is a delay from the time charges are incurred to when they show up in Cost Analysis. What this means is, after deleting your Azure Database for MySQL Flexible server the cost shown in Cost Analysis will continue to increase over the course of hours and then finally stop.
Typically the cost shown for the deleted resource in Cost Analysis will stop increasing within 24-48 hours after deletion, and often with stop in fewer than 24 hours. How long has it been since you deleted the database server?
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Thanks.
-TP