Thank you for the response. Based on your observation above I think to resolve this issue we will require a support ticket as a support engineer can take a look at backend logs and determine the exact cause and help you with the DNS label transition. Meanwhile I will request you to keep the DNS label assigned in your old tenant, just in case if the label is unassigned and if it is taken by any other customer then it won't be recovered. Please follow the instruction from support engineer for further actions.
Please let me know if you do not have a support plan, I can help you with a one-time free support in this case. Thank you!