For the last week or so, every time Mail.app launches, SpamSieve asks for permission to access contacts. I always grant permission, but the next time Mail launches, SpamSieve asks again, ad infinitum.
I had only modified it by hiding its icon — but not via the Terminal method.
For the benefit of others, the first Terminal command failed to work until I replaced the app with a freshly downloaded copy. All good now. (Fingers crossed.)