Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
This happens when a VNET Integrated resource gets deleted before removing the VNET Integration.
In your case, it appears to be a "Microsoft.DBforMySQL/flexibleServers"
We connected offline and a Support Engineer was able to delete the SAL post which you should be able to delete the subnet.
Kindly let us know if you need further assistance on this issue.
Thanks,
Kapil
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