JOIN LINE AND POLYLINE AND IT CREATES A SPLINE? WTF?

Joining a line and a poyline containing a polyline arc creates a spline now? I don't believe I've encountered this behavior before? Why can't it just create a polyline? If I convert the spline back to the polyline it now has 1000000000000 vertices in the arc, instead of a standard polyline arc? Is this a variable somewhere? Or is this standard and somehow I missed it the last x number of years?

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