Hi @Werner the problem there is that myvar contains the title of the data tiddler. The json operators require that the JSON data itself be the input data, not a tiddler title. This is done sot that we can manipulate JSON within variables as well as tiddlers.
So one way to fix the code would be like this:
<$list filter="[tag[myTag]type[application/json]]" variable="dataTiddlerTitle">
{{{ [<myVar>get[text]] }}}
{{{ [<myVar>get[text]jsonget[myJSONkey] }}}
</$list>