Whats up with my left shift key?

I bought this MacBook on eBay and ever since I got it, it's had problems with the left shift key, as in it was stuck down/on and today I replaced the keyboard thinking it was going to fix the issue, and lo and behold, nothing happened. I still have to use the right shift key anytime I type and press option when I boot so it doesn't go into safe mode. This isn't a huge problem to me, but I still just wanted to see if anyone knew what was wrong with it. I don't think it's water damaged (as in there's no corrosion on the logic board, but maybe there's another sign I'm missing? idk), and I have no clue what else it could be. Thank you so much for your help!!!