In this case, the spline mesh component doesn’t update when the start tangent is changed because it isn’t calling the correct update function internally.
You can work around this by adding an event repeater track that calls UpdateMesh for each frame. You’ll want to set the start/end range of the event repeater section to only the range that you animate those values. You also need to check Call In Editor to see it update in the editor.
Here’s the docs on the event repeater track: