This was a known issue with the Domain Verification which has now been mitigated.
It turns out the reason this issue is occurring is because the content type header needs to be exactly :
application/json
Not
application/json; charset=utf-8
or some other content header type.
Suggest you to please try this and let us know the result.
This
is now documented in the link below.
https://docs.microsoft.com/en-us/azure/active-directory/develop/howto-configure-publisher-domain
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