Diagnosis
The auto-off timer mechanism has malfunctioned. This is a safety feature failure, not just an inconvenience.
How to Fix It
First, verify the feature: most Hot Tools irons have a 1-hour auto shut-off. Time it precisely. If it definitively stays on longer, unplug it and retire it from use.
If That Didn't Work
There is no user reset or repair for the auto-off circuit. The unit is now unsafe for normal use. You must replace it.
Heads Up
Treat this iron as if it has no auto-off feature. You must manually unplug it after every single use. This failure significantly increases the risk of a fire if forgotten.