Can you try out the steps mentioned in the below discussion?
https://stackoverflow.com/questions/73325006/unable-to-delete-restore-point-in-azure-backup
And this link also has got detailed troubleshooting notes and different options:
https://learn.microsoft.com/en-us/answers/questions/964585/unable-to-delete-restore-point
There is a good amount of explanation of what happens behind the scenes.
Let us know how it goes when you follow the first post & Second post
Regards,
Shiva.