"Under many conditions the WPB layer will trap moisture which will condense and wet out your bag, before it can escape. Basic physics. It can also turn to ice ... This happens especially when the outside is rather humid, or very cold.
Under some good conditions the moisture manages to escape through the fabric before it condenses, so you can be lucky. For it to escape you want a moderate temperature, a little breeze and a low external humidity.
Physics didn't seem that simple in college. :^)
Moist air holds more warmth than dry air.
Outside moisture can quickly overwhelm a DWR coating.
Sudden heat loss can occur by evaporative cooling.
There are no "one size fits all" situations.
Sometimes I carry a DWR Bivy, sometimes a WP/B one,
sometimes an insulated overbag, sometimes a VBL,
many times nothing over my bag, sometimes when it
is hot and buggy I use just the bivy, sans sleeping bag.