RobinCavalier-1802 - Welcome to Microsoft Q&A and thanks for reaching out to us.
As Yegu has mentioned, rephrasing it here for broader community that these are publicly facing static IP addresses allocated by the underlying VMs. If your Managed Grafana instances aren't deleted, the same IP addresses won't be used by other instances. Hope this helps. and please feel free to reach out if you have any further questions.
If the above response was helpful, please feel free to "Accept as Answer" and click "Yes" so it can be beneficial to the community.