Since your Symbol is a Group, I would use "Edit / Edit Group Content" and see if the objects within the group are also set to zero. You can change their line width there to whatever you want. I like to set line width's to "By Layer", then specify a line width in the Layers dialog. When finished editing the Group, use "Edit / Finish to Edit Group" to return to the main drawing.
If you have several layers with different Line Width settings, you could place a copy of the group on each layer and its line width will be different in each case. I hope this isn't too confusing.
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Windows 10 Pro (1909), 64-bit