hello tranhung,
what is the cause of HO failure G900 to G1800 ?
if it is congestion, then i would say it is normal. the preferred band HO will push MS from G900 to G1800, leading to HO failure due to congestion. It is a normal behaviour, a “wanted” behaviour (preferred band HO will push the MS…)
if it is “radio failures”, then i would check your HO tresholds, especially the l_rxlev_cpt_ho and the traffic condition (any load ?) to perform the “preferred band ho”.