The reason for this is that only the Scatter trace type accepts spline for line shape, Scattergl doesn’t.
If you use plotly express to create your figure, the default is to automatically switch between the two trace types depending on the number of points to render (above 100 points it picks Scattergl). You can change this default behaviour with the render_mode argument, set it to “svg” to draw spline with higher number of points.