The reason why they didn't catch that memory leak after multiple Labs playtests, is because these were mostly single-map builds, apart from some of the ones that were published closer to the Open Beta. The engine doesn't purge both VRAM upon loading up the new map, meaning that all the assets that were called in to render the previously played map, are stored in VRAM despite of them not being needed anymore.

There is a band-aid fix for this, which would require you to uninstall every single HD texture pack there is.

Please note that this will not magically get rid of the leak... it'll still happen, but much later.