Cruise mode activates when the differential pressure is within 0.25 psid of the target differential pressure. If you stopped climb at FL380 and then reset the cruise altitude on the panel to FL380, the "automatic abort capability' is lost. Note that Boeing's recommended way of handling step climbs is to set the highest expected FL on the ground, and then as you step climb, the pressurisation controller will take care of the rest. If you divert when at cruise level already, you should re-set the landing altitude on the pressurisation panel.