PSA - Don't use the "Promote Post" feature on a post that's already paid out once. Only use it for new posts.
For the time being, DO NOT promote a post that's past it's first payout phase.
I've opened an issue on github about the issue and hopefully we'll see a resolution soon.
Currently you can promote any post, even those past the first payout phase. The problem is that if it's in it's second payout phase or beyond, as soon as someone votes on the post, the promoted status gets completely removed.
I'm putting this message out there to hopefully help avoid any potential frustration this issue may cause.