This is not a problem in STAAD – rather, note (b) of clause 26.5.3.1 is followed in this case.
In this case, STAAD determines the total cross-sectional area to be larger than that required to support the load. Hence, note (b) of the aforementioned clause is applicable, which says that if the concrete area required to resist the load is less than the cross sectional area, the minimum reinforcement should be based on area of the concrete required rather than the gross cross-sectional area
Please refer to the following discussion:
Negative axial forces indicates that the column is under tension.