I have just posted a scheduled post to my blog for publication a days hence. It doesn’t show in the blog as expected.
However it has federated to the Mastodon timeline and is visible from there with its publish date as 8th March (today is the 7th). Is this a BUG…? I don’t know…!
Yeah, this is an open issue right now. The bug is with the scheduled post being federated (this shouldn’t happen at all), but we also need to build in the processes to actually federate scheduled posts. That should all be coming soon.
Knew you’d be on it 
Just for the record because it may help you finding a solution, I have the exact same problem and noticed a scheduled post that popped up in my Mastodon timeline is labeled as posted “now” with a date in the future (June 26), but is chronologically below a post that I actually auto posted via Buffer an hour ago (June 1). I don’t remember when I edited the WF draft.