I finally experienced this for the first time last night (after 20 or so patrols over two careers) - when I saved and quit the other night it was storming 15m/s with heavy rain, which I could see just fine and my Nav reported correctly.
When I loaded up that saved game last night, it was still storming 15m/s and raining hard visually, but my Nav reported "no rain" - that went on for a day or so, but after about the third time I dived for a 'phones check and then resurfaced the Nav started reporting the heavy rain again. Although I've seen rain in storms (and storms without rain) lots of times over all those patrols and my Nav has always gotten his reports right up to now, this may have been the first time I ever quit/saved while it was raining - it's got me wondering if this bug may have something to do with the problems the game has in retaining saved game states sometimes (I've learned my lesson about never saving while in close proximity to the enemy, after having been perfectly safe and undetected when I saved and being DC'd and dead before I even finished loading into the save game when I came back).
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