They can release iOS 11.1.2 just for the iPhone X to resolve the cold issue, it doesn't need to be on all devices...
Again, they need to figure out
what causes the issue. Is it a hardware issue (than no iOS update can fix that) or a software issue? You don’t find that out within days (that could take months).
Than you need to fix it (if it’s a software issue). Even than it might not be fixed at all. The problem has been present with earlier iterations of the iPhone that it refused to work under certain weather conditions. Typically in the past the issue was on the hardware side which forces Apple to change the device.
The smaller the sample size the harder it is to figure out the problem. If it was a larger issue, it is easy to reconstruct the issue and solve it.