appendSeries and change the dataLabels to return a custom string

[amanirath4]

I am rendering a timeline advanced(multiple range) chart with no problem. Next I am appending a new series and need to change the dataLabel on them. The appended new series renders on the chart but the dataLabels is not displaying what I am expecting. The dataLabel is displaying the data of the format in the initial option settings.

Is this possible what I am trying to achieve?
Here is my appendSeries snippet.
chart.appendSeries({ name: "Testing", color: "#06A9EA", data: [{ y: [1648425600000, 1651190400000], x: "Testing Program" }], dataLabels:{ enabled: true, formatter:{function(val, opts){ return 'Test Custom Label' } } })