Thanks for your update.
We are afraid that there is no option to check the TextAnnotation is placed on top of the LineSeries.
As we can’t find out text annotation on line series, we need to check the data points and text annotation position and will need to adjust it manually using the CoordinateUnit axis or pixels.
As a workaround solution, the DataMarker can be used at a specific point as TextAnnotation and can hide the remaining data markers using DataMarkerLabelCreated event.
We have prepared a simple sample using the DataMarker as TextAnnotation to show the text without placing on LineSeries. By using the LabelPosition as inner, outer and center you can also change the text position for the DataMarker.
Please refer the below UG link for more details to position datamarker label
Please let us know if you have any other concerns.