People still own the 17" ? it’s a big boy that one! What OS you running on that puppy?
The keys are a nightmare on all the Macs to be honest.
It’s the white butterfly/Scissor mechanism under the key that is possibly causing the issue but without seeing it It’s hard to call.
The mechanism can come loose and need clipping back in and then the B key sliding back in too.
Sometimes, and I know this sounds silly but it can be the crumbs from that Pret sandwich you bought for lunch last week that crept in under the keys and just needs blowing out.
the worse thing I found under a key was a small dead spider from when a customer of mime went to Africa travelling. Luckily for me…it wasn’t a black widow!
Only recently did Apple redesign the keys and keyboards across the range.
They even had a repair program running not long ago where customers were coming in and having all the keyboards replaced (top case apple call it)
That’s the good thing about Apple. They admit a fault and then get to work sorting it out no questions asked.