There are TWO DIFFERENT mechanisms for automatic life jackets:
1) a mechanism that uses a moisture/water-sensitive cartridge, at the bottom on one of the sides of the “horseshoe”.
When this gets sufficiently wet (normally submerged but, potentially, if you leave the jacket under soaking waders, jackets etc), it goes off (with a surprising bang). This is where the “it went off in my car” bit comes from.
Clearly, if you are wearing the jacket, the water 99.9% of the time flows off without touching the cartridge, so you SHOULD be all right in heavy rain....
This mechanism is very reliable (i.e. it goes off when you go under), and is the cheapest automatic mechanism: it is what almost all ”fishing” automatic waistcoats use.
2) the pressure-activated (“Hammar”) type. This is totally insensitive to moisture (so it works really well, e.g. on a RIB, where you can get drenched in spray). And you can take it off and dump it under totally soaking kit etc if that is how you end your day. But, if you fall in and the mechanism is submerged approximately 1 metre, then it inflates. A bit more expensive (typically c.£20 more), but many fewer false inflations. These are what I use on my RIB and, were I to fish big salmon rivers, what I would take. But the vast majority of “fishing life jackets” don’t use the Hammar mechanism, so you need to get them from chandleries and marinas.
Manual inflating life jackets have none of this complication, but are horribly dependant on you being conscious/not sufering from cold shock/actually able to find the toggle to inflate the jacket. Cheap, but may not save your life in the most likely scenarios that we might want to use them. I would never use one for the sort of fishing I might do (RIB, big river wading etc): they give an utterly false sense of safety. Probably OK on a reservoir/lough, though.
My 2 pen’oth.