It is a physical problem with the key. You can fix it by disassembling the key and cleaning the contacts:

Original guide - https://imgur.com/a/elAFF#0

Thread with context - https://www.reddit.com/r/MechanicalKeyboards/comments/2hjct6...

However, this is really Matias's problem to resolve. Supposedly the switch or the assembly process has been redesigned to prevent this problem in the newest batch, but I don't know if it's a proven success.