Legend: Tenacity Bug

Don't have a screen shot or anything, but Legend: Tenacity does not seem to want to cooperate with other tenacity effects, in particular Mercury's Treads.

With all the stacks of the Legend: Tenacity rune and Mercury's Treads, a person should be able to get 50% Tenacity. But that's not what I got, I had a total of 43% tenacity listed in my champion details in-game screen.

Now if that number sounds weird, it is. Not only is it the incorrect amount, it's also an amount that should not be attainable at any point; the Tenacity rune gives 5% tenacity immediately and 1.5% per stack for ten stacks. Additionally, Mercury's Treads gives 30%. There is no possible mathematical way to work out those numbers to get 43% tenacity, at all.

The closest possible justification would be to say that the legend rune is rounding and giving 13% rather than 12.5% when someone has 5 of the 10 stacks. But that wouldn't make sense because I had already maxed out at all 10 stacks, so the rune should not be trying to give me only 12.5% anyways.

I've not yet tested it with other tenacity giving effects, such as champion abilities or passives. But given I found this by random chance in one of my games, I'm not going to bet the other interactions are not bugged as well.