F3 + B does't show hitboxes anymore

When i press fn + F3 + B nothing happens. This is really annoying when making maps.

What I expected to happen was...:
It should be showing the hitboxes.

What actually happened was...:
Nothing.

Steps to Reproduce:
1. Join a world (The world i got the bug was a Flat world, i did't tried on other worlds).
2. Press fn + F3 + B