I use mdCharm to format and save before posting, just in case, but I've never stumbled on a lost post. My son has, though, and I'm pretty sure I've seen the same thing with some of my votes. I also used to see it when mining was enabled. My miner would get 1 or 2 entries into the miners' witness list, then disappear.

With mining, I always assumed it had to do with the consensus process. i.e. I got on a short chain that got replaced by someone else on a longer chain.

Not sure if the issue is the same for posting and voting (and transfers?). You would hope that the web site would hold a copy of the the transaction until the witnesses all reached consensus - then resubmit if necessary.