Previously you could hold tab and tap an ability key to unlock or upgrade it, but this does not do anything (aside from use your ability if it was already unlocked) after the latest patch.
Yeah, I've also been experiencing this. It's been somewhat annoying since I got pretty used to holding tab and pressing the corresponding number key in order to unlock abilities when playing.
It seems like this is mostly fixed now. The only time I can't hold tab and press one of the number keys to upgrade an ability is when I'm dead and awaiting respawn (Build 4360).