The ? on my magic keyboard isn't working, but the / on the same key is. What on Earth is wrong?

I purchased an iMac 2015 secondhand. The keyboard was working find when tested. When I got it home I upgraded the operating system to the lastest. Now the “?” isn’t working, but the “/“ on the same key is. I took it to the Apple store and it worked when the man connected it to one of their new MacBooks. When I got it home is again wasn’t working on my iMac. I paired it with my MacBook and the “?” works only sporadically. Is it a coincidence it stopped working with the operating system upgrade? I don’t want to spend $140 on a new keyboard because one key doesn’t work, especially if the keyboard is working with some other devices.