It's the Google Authenticator app's fault. The most popular TOTP app probably, and for a long time, they were saying it's intentionally designed not to let you copy the codes. Now you can, but there are lots of pitfalls and vague documentation. I'm not convinced that TOTP is a user-friendly design to begin with, but it didn't have to be this bad.

I don't fw TOTP now. There are other apps, but I'm done. I'll only use it if the iPhone Keychain has built-in support some day.