For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
Impossible to have only one key not working because of MLB issue.
The keyboard matrix has more keys connected on every raw and column.
Check again all the keys, you should find other non working ones; maybe special functions.
Turned out the issue was U4820. Simply reflowed it with some fresh flux and it solved the issue. There must have been some liquid under it. Originally the Logic Board was liquid damaged but showed no liquid damage near the keyboard connector. Anyways all is working perfectly.