Your spoiler tag works fine on the web client. That is the supported way to do spoilers on Lemmy. However, Sync, if you use that doesn’t support Lemmy spoilers and only supports reddit spoilers. Unfortunately no matter what spoiler method you use, it will not work for someone, somewhere. There is no unified spoiler markup across the fediverse, but there really should be.
The tooltip for fedipact says: “Agreed to block all communications (their blocklist is private)”
To me that says, they’ve agreed but it’s not confirmed that they’ve gone through with it because the blocklist is private. Blocked on the other hand says “All communications are blocked”