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Although, I'm not sure, but you may be able to comment/upvote via the API without actually opening the Steemit page, so if it is keying off the interaction with the application, that could be happening too.

Still I tend to think it is a bug, an update delay!

Yeah, definitely a possibility. I imagine it could be related to the amount of data in the post (comments, etc.), since I know that I've seen issues with the 'wallet' page when there are a lot of transactions. :)

Yeah, definitely a possible bug. As I posted it I realized that could be at play too, so have updated the article.