You may have a bad valve.
Or you may have a bad expansion tank and a good valve.
Or both may be bad.
Or the expansion tank may not be all bad yet, but might not have enough air in it. Since it SHOULD hold air pressure that's not a great sign, but a careless install might have left it with too little pressure - but if it's not been leaking for 9 years and started leaking now, that points to something changing now, which would mean a leak if the air pressure is low (but usually any leak eventually results in water on the air side.)
Your troubleshooting so far does not provide data to disambiguate which has failed.
A peak recording pressure gauge would show if the pressure is or is not going above the setting of your relief valve. If it is not, the valve is bad. If it is, the valve is just doing its job.
Shutting off the water heater, and the water supply to the water heater, and releasing all pressure on the hot water system would allow checking the pressure in the expansion tank. A quick blip of the Schrader valve with a tool (screwdriver or similar) before releasing pressure on the hot water would let you know if air or water came out (then you'd release pressure on the water side and check/set pressure on the air side, if not failed.) If water comes out the air valve, you need a new expansion tank. Don't ruin an air gauge by putting it on before checking for water. Per your measurement of "normal pressure", it should be about 70 PSI air with no pressure on the water side for best effect.