@Nathan Hofstee Thanks for reaching out. There is an ongoing issue at service end where DELETE requests with api-version 2023-03-01-preview to APIM are returning 400 Bad Request with error message 'Invalid payload format. Contract should have 'properties' specified'.
Sorry for the inconvenience caused by this issue. Our product team is actively investigating and we don't have any ETA/workaround for this. once we have any updates, I will update you here.
do let me know incase of further queries, I would be happy to assist you.