Input data is not a valid JSON

Input data is not a valid JSON

I am trying to create accounts under a resource using the REST API. This is the JSON that I am using:
  1. {
      "operation": {
        "Details": {
          "ACCOUNTLIST": [
            {
              "ACCOUNTNAME": "Account_Name1",
              "PASSWORD": "XYZ12345#",
              "NOTES": ""
            },
            {
              "ACCOUNTNAME": "Account_Name2",
              "PASSWORD": "XYZ12345#%",
              "NOTES": ""
            },
            {
              "ACCOUNTNAME": "Account_Name3",
              "PASSWORD": "XYZ12345#%ee",
              "NOTES": ""
            }
          ]
        }
      }
    }

This JSON block fails with the error "Input data is not a valid JSON".  Look at the password for Account #2.  If I remove the trailing "%", or add something after "%", then this block of JSON is successfully parsed and the accounts are created.  Why?

Version 9.1.0, Build Number 9100.

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