Under certain conditions, the buckling analysis may show negative values of the buckling factor.
The buckling factor with values smaller than zero means the stress acting in the plates is induced by tension forces. The negative buckling factor actually means that there is no buckling in the connection and the program only informs you about the buckling factor values and shape.
In case there is a load combination with inverse directions of internal forces, this is a warning and valuable information of a possible buckling problem of the connection.