Why in Adobe XD a normal vector element (Rectangle) at some point becomes an outline (Path)?

Answer from: Elsa M.:
Hello, hello!. :) I am an optimist by life, a naturalist, a gardener...

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The problem you describe can only occur when the function is activated "convert into a circuit".
Hotkey operation ctrl + 8, or in the dropdown menu (when right-clicking) item "Contour" > "convert into a path".
I don't know of any cases where XD has converted shapes into paths by itself.