It looks like you're new here. If you want to get involved, click one of these buttons!
While doing Copy to paste from AutoCAD polygon entities to BricsCAD its not showing as polygon entities, its show as OLE frame, it should be as polygon entities
If you are talking about an MPOLYGON: I have no problem copy-pasting such an entity between two drawings in BricsCAD.
... And if by 'polygon' you mean in fact 'polyline': Copy-pasting that is, of course, no problem either.
I believe OP is talking about COPYCLIP / PASTECLIP from AutoCAD -> BricsCAD. These are two independent applications, BricsCAD can't (or doesn't) access the clipboard format used by AutoCAD, so pastes the objects as an OLE. Same will occur if you try going from BricsCAD -> AutoCAD.
Copy & paste will work if you have both drawings open in BricsCAD.
I did consider that interpretation of the OP, but then thought it a bit far-fetched.