It was discussed before, probably many times, but when a filter is prefilled, oil goes through the media. I just prefilled one so I know this is true. So the reverse has to be true, oil will also flow through the media into the center tube and out. But filters are seen to hold oil when they are turned upside down. The center tube drains but the part behind the media doesn't sometimes. The difference is when prefilling there is air pressure above the oil going in. If the bypass valve is leaky it can cause draining of the oil, it isn't just the adbv only to blame. The oil will drain out if air is above the oil, just like when prefilling it goes in, even with the world's most perfect adbv.