You can expect detected Max HR to change over time due to improved accuracy from usage, but a HR that high does seem suspect and I would assume a software issue. Real underlying MaxHR shouldn't increase significantly, and it's expected to decrease with age.

If you're not already using a heart rate strap, you should get better accuracy that way (there are also armbands that I hear are quite accurate too). In addition, Max HR based workouts are the least accurate anyway and if you use a strap, you can do lactate threshold based zones.

Firmware upgrade and/or a factory reset of the watch may help the actual problem with the detected MaxHR, though.