Hi @Frederick Wong ,
Thanks for bringing this to our attention. This is odd behavior. Are you getting any errors? did already you bind your cert to a custom domain in the old resource group?
As the doc mentioned "You can't move a free App Service managed certificate. " If you don't need the old resource group try deleting it and create a new free managed certificate in the new resources group.
If you have further questions. please let us know.
Hope that helps.