Answer
This has been reported and tracked as a genuine platform bug (not a permissions or configuration issue), affecting Copy/Paste Rows specifically in certain windows while the equivalent functionality still works correctly elsewhere (e.g. the Design Objects window). The Duplicate button is not an equivalent workaround — unlike Copy/Paste Rows, Duplicate doesn't support bulk multi-row copy-then-edit workflows (like copying a structure of many records and then adjusting a shared value across all of them) or copying rows between separate environments/databases, both of which genuinely depend on Copy/Paste Rows working.