You could try removing the SMS permission from the app, and just type the code into Signal manually once you get the SMS with the code.

I doubt you're actually having this problem, but if you are, you can add an issue to the corresponding Android/iOS repo.

They had announced capacity issues with SMS service due to onboarding a massive number of new users.

That resulted in delays, not incorrect codes.

Maybe the code was taking so long to get through that it was expiring?

At any rate, I know a couple of people who have recently signed up so it's not affecting everybody.