I can’t speak for all inflatables, but I’ve been using the Float-Tec brand for over 6 years now while being on the water whether guiding or pleasure boating.
I can’t locate the photos right now, but two photos of my clients made it into on of the Lund Boat catalogs a few years back. They were soaked. That happens frequently when fishing in the rain. 
As long as the water doesn’t come up from the bottom of the PFD it won’t discharge and inflate.
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Vest only pictured above.
Normally if a person is getting wet they would want to have some type of jacket on. Nothing can be worn over an inflatable. If it would discharge it could prevent a person from breathing.
Float-Techs can be purchased with an “outer shell” that’s made for the vest. The shell will expand if the vest inflates.
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The outer shell is a light nylon wind breaker style with a hood rolled up in the back.
Either way, I have never (I hate saying never on the river) had a Float Tech discharge in the rain.
Full disclosure: Since no one in the area stocks them, I’ve been set up as a dealer for them in the last 6 months.