A few years back, I got the congratulations message for achieving Exploration Elite. I was the additional decal was added. I then took a bit of a break from the game. Returning, I was no longer Exploration Elite - though I was really close - and the decals had gone. I raised this as an issue, but even my game logs seemed to "cut off" before my achievement, suggesting I was close, but not quite there yet.
General consensus was that I just imagined it, or got the achievement during a Beta - I wasn't part of any beta - but I was never sure exactly what happened. My friend, who I regularly wing up with, remember me telling him I'd just achieved this milestone, so it wasn't just me who imagined it lol.
So, yes, once before I had what must have been an incorrect congratulations message, which seemed legit, but weeks later, it was like it never happened. Have had similar things when doing the Fed / Imp rank grinds, get the promotion mission, do the promotion mission, remain the same rank. That was a long time ago though.