I feel that the output transistor is damaged.
If you have put a less value of pull up resistor, then at output low, the transistor will take more current. Depending on the value of the resistor, this can be any amount. You should select the resistor value that would keep this current below the rated value.
You can calculate this by R = (VDD - VOLmin) / Current.
Select Lower current to prevent damage. Higher current would give better speed but can also cause damage due to heating.