What is the geometry of the layer you're adding? Polygons are added underneath lines and lines are added underneath points. But note that if you want to add a layer in a specific drawing order, you can drag and drop it into the Contents pane right at top.
This might be a work around but not automatic.
Have the Map and Layout views of interest active.
Turn off New layer for the Legend Element
Add the new layer to the Map --> drag it from the Map's Contents pane and hover over the Layout tab to open the Layout view --> drop the layer onto the Legend element (there should be a small grey rectangle by the pointer when over the target). Layers are always added to the top of the Legend in my tests, regardless of feature type or position in the Map Contents pane. You can of course manage the order in the Layout's Legend group layer.
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