I discovered recently that there was a change made in SQL Server 2017 to the way that symmetric key passphrases are hashed. There’s a KB article that notes the fix, but basically the passphrases used to be encrypted with SHA1. That’s cryptographically insecure, so the algorithm was updated to SHA2.
This is a problem, and can cause some issues. I’ll show the issue and then how to get around it.
There’s not much detail in the KB article about what happens afterward: if you need to keep the trace flag on forever or if it eventually migrates everything over to using SHA2 for hashes.