Your coders are correct, and a 7th digit is NOT required. Z00.110 is the highest level of specificity, and no additional digits are required. I don't subscribe to craneware.com that you mention, but both my Optum360 ICD10 book and supercoder.com state this. Of note, this is a code that is noted for primary dx only, yet unacceptable principal diagnosis for inpatient admission per Medicare Code Edits. From my ICD10 book:
NOTE: Please note that the symbols indicating the Z code "principal or first-listed only" designation & the Z codes that may be principal or first-listed diagnoses included in the official coding guidelines are consistent with reporting guidelines for health care encounters EXCLUDING ACUTE CARE INPATIENT ADMISSIONS. These Z code edits are often in conflict with the inpatient prospective payment system [IPPS] edits. For example, Z34.90 encounter for supervision of normal pregnancy, unspec, unspec trimester, may be an appropriate primary reason for an outpatient encounter. However, supervision for a normal pregnancy is not an acceptable principal diagnosis or reason for an inpatient admission and will have an unacceptable principal diagnosis edit under IPPS.
Your denial may be related to this, as there is no 7th digit for that code.
I hope this helps.