Wasn't Muramana Ezreal supposed to scale well?

I thought Muramana was supposed to let Ezreal scale well late game since it's not going to be complete until ~20 min. However, even in 35+ min games with this build, he has the 2nd worst winrate in the game for an ADC. This is even true in Diamond+ where his pick-rate is much lower and you'd expect higher skilled players. His highest winrate in all leagues as an ADC is between 25-35 minutes. His overall ADC winrate is 47% but it jumps to 49% during those 10 minutes.

Anyway, I suppose I'm looking for a way to make Ezreal more competitive late game and I expected Muramana to be it. In the end though, the statistics suggest he still falls off pretty hard after 35 minutes. I really like his kit but my results are very inconsistent.

BTW: Does anyone know if Muramana's toggle gives an additional Black Cleaver stack for Ezreal's Q? I've been considering BC on him since his tank busting power is fairly weak even with LW and BoRK.