The plausible explanation is, that we used excess (4.5 equivalent) of lithium bromide for preparation of CSBr2. In case of disubstituted diazo, the bromide ion is more reactive then CSBr2, to give the gem-dibromo product, whereas in mono substituted diazo reacted with CSBr2 and gives the expected 5 membered ring. CSBr2 is indeed formed in the reaction, it incorporated in product 4c.