Unless I haven't found a documentation for it, there's a bug in API processing when an array of records is send to API endpoint: domains/<DOMAIN_NAME>/records with a PUT request.
If the array contains _domainconnect record, the API returns code 422 (duplicate record).
I'd expect that this record, just like any other records, will get overwritten without a error code.
Issuing PUT with an array in which other '_domainconnect' record is removed fixes the issue
W. Adam Koszek