A lot of people don't seem to know this but California is a vast state with practically every single kind of climate and an incredible range in altitude.

I don't have enough experience in California to say just how cold it gets in some parts but it certainly does snow in mountainous areas.

Most major automotive and ev companies use tools such as environmental chambers to do phases of R&D. (quite possibly all do but obvs can’t confirm, but do have first hand knowledge of several major companies adopting this) Even companies much smaller than Tesla.

Would be shocked if Tesla did not have this capability.

So who knows if this is part of their testing and validation, but it is not true that you can’t test for different environments than where the core design takes place. It is extremely common to do this in rapid fashion without having to transport things to an alternate climate.

Are engineers more thorough and thoughtful in Michigan?

I'm confused on the point you are making?

What I am getting out of your comment is that their weather doesn't go below freezing (which as a sister comment pointed out, California has plenty of places that are cold and snowy), which was my point. They don't have a test plan to include testing their cars for regular usage (I would think plugging in a car charger outside of a garage qualifies as normal usage) in weather conditions that a significant portion of the US population in fact have (weather conditions below freezing and humidity), but such weather conditions are not in their area.

What else is their test plan missing then?