MacBook Pro Shift key problem
I have the same issue with MacBook Pro A1502 EMC 2875 on macOS 10.12.6 fresh installed and previous 10.10.

I also see graphics glitch on PWD logon switching to the desktop, and it's sluggish.

I re-seated ribbon cable and reset SMC. It worked for a while, then it returned.

I believe the keyboard got light liquid damage, so I'm changing it as a test.