After routing the initial layout I have came across some antenna design related material namely the AN91445 from the Cypress. Section 7.1 covers the YJ-14015 antenna type (MIFA == Meander Inverted-F Antenna).

By seeing this radiation pattern:

The current orientation is not the best (most likely the keyboard is going to be used in front of the receiever).

It's time to update the design.