Too early HO maybe: UE moving very fast, UE report target cell is better, after HO procedure complete and UE HO to new cell, but at that time, signal of target cell already changed, not good as the time UE report → RLF on target cell.
Too late the same.
UE must support SN 18bits. If not, enb/gnb SN > UE SN, maybe pdcp/rlc packet loss