High-voltage amorphous indium-gallium-zinc oxide thin-film transistors can be monolithically integrated into the system-on-chip platform as input-output bridges. However, a transient instability showing substantial ON-current degradation under high drain bias is discovered. This drain-bias transient instability depends on the stress time of less than 10 s and both drain and gate stress voltages. It is attributed to the charge trapping in local oxygen vacancies with shallow energy levels and the migration of oxygen ions near the drain. We found that oxygen vacancies are induced by metal contacts. An elevated-metal structure suppresses the transient instability by separating the metal contact region farther away from the channel compared with the conventional top-contact structure.