How do I get the actual shape instead of just my path with the dynamic shape somehow untouchable in the background? I’m thinking something like Illustrator’s “Expand” command. I tried context menus, reopening the Filter, View menu, Path menu… how do you work with filters like this?
My guess is that you have View > Show Preview Offset checked. So, the grey shape is showing what would happen if you selected Filter > Offset Curve with the last options used in that filter.
Select the Offset Curve filter from the Filter menu and make sure to press the Offset button in that pop-up panel. That should create the shape.