All membranes, no matter what brand or construction, will block water but can let water vapor through. But it will only do so under certain circumstances. Let’s forget about the differences in specs and quality of the membranes for a while and concentrate on the principle.
The key word is difference. There will only be migration of vapors if there is a difference. On one side it should be more than the other. The gas or vapor will want to go there where the least vapor is, until it is the same everywhere. The membrane however has no idea about front and back, so it can go both ways.
Always keep in mind that there are circumstances and effects that could let the water go inside your gear instead of out. If it is more humid and warm outside, than inside your gear, vapor will be drawn in. This could also happen on your seat, where water remains, and temperature is higher. You might get the idea your gear is leaking, but in fact it is just your membrane doing its job.