Here's roughly what currently happens:

If the right column has a bound key or button, a new key or button that you attempt to bind in the left column will be removed from the left column and combined with the right column's existing binding as a chorded bind.

If the right column is empty, a new key or button that you attempt to bind in the left column will disappear from the left column the first time you click Apply.

Repeating the bind-Apply sequence will work the second time.

If you close the options, the state of the code is reset. So many users go through this sequence several times before they figure it out, or give up altogether:

Bind a key. Click Apply. Exit options without realizing that the new bind disappeared after clicking Apply. Test bind in-game and realize that nothing happens. Open options again and repeat sequence.

Please note: this broken behavior is immediately obvious upon the most simple attempt to modify keybindings. The most cursory test of the keybindings UI would have revealed it to the developer before release.