Verification is nothing to do with decryption - verification is how you confirm that you're really speaking to the person you think you're speaking to.

What you're describing sounds like a plain old encryption bug, although I'm very surprised you're still hitting it, given encryption in Matrix is pretty stable these days. If you can file a bug on github.com/vector-im/element-web/issues (or element-ios or element-android, depending on what platform you're using) with the full details or reproduction steps, I'll take a look.