I2C issue with 3.3V MCU master and 5V powered IC slave
The chip cannot be used on a 5V I2C bus when it is powered with 3.3V.
The maximum on any pin is the supply voltage, so when powered with 3.3V, the I2C pins are not 5V tolerant, and the chip is used outside the specifications listed in the datasheet.
As the current is limited by the pull-up resistors, it is possible that there is no permanent damage to the IO expander, as maximum IO pin voltage is VDD+0.6V and maximum IO pin clamping current is 20mA.
Also the IO expander cannot be used on a 3.3V I2C bus when powered with 5V. The minimum logic high voltage is 4V, and 3.3V is not enough.
So the IO expander supply voltage must match the I2C bus voltage, regardless of what voltage levels the MCU supports.
Adding scope probe (or a finger) adds resistance, but also capacitance, so perhaps there is a problem on the pull-ups, either bad soldering or just incorrect value resistor. Check that the pull-up resistor values are reasonable for the I2C speed you are using, and that the rise and fall times are not violated, or other timing parameters.