Actually it just cant be found why this doesn't work for you unless you provide a snippet replicating the issue.
Some of the possible reasons could be,
- You're making a typo. (model set as -> MyJSONData & you're accessing it as MYJSONData as per the question)
- missing out data-sap-ui-xx-bindingSyntax="complex" in the bootstrap script.
- somewhere in the code you're setting the model MyJSONData to the core again and it gets overridden with new data.
