I’ve been trying to complete Glory of the Pandaria Raider and just ran Mogu’shan Vaults on heroic , but only the normal achievement completed. I double checked it to make sure and it was set for heroic.
Also – when I tried to enter the Vaults – instead of it letting me enter normally, it ported me to the middle of the dungeon – still with all bosses and adds killed. Don’t understand what is going on.
Likely some shenanigans cause of how the older lockouts work now. It is possible to get the heroic achievement you have to do it on Mythic or as the 25 raid size and as far as the port, all lockouts could be shared so if you did it on normal, it counts for heroic.
Except for Siege of Orgrimmar, the Pandaria raids work on the boss kill lockout system where you are only allowed to kill each boss once per lockout period. If you’re soloing these old raids, this means once you kill the first boss on normal mode you’re locked into normal for the rest of the raid; you can’t enter the heroic raid because the boss you killed is still alive & you’re not allowed a second kill of that boss. So you have to think about which version of the raid you want to run each week before you start, because you can’t change your mind after that first kill.
Also, some of these raids you had to start the fight in a particular way to get the heroic version, so make sure you’re doing that as well.
That was in Ulduar only as far as I know, everything else either was a swithc on the raid difficulty or did not have a “heroic” mode.
Thanks very much for the response. 
I, unfortunately read it too late. Just ran a heroic run for the Straight Six achievement, which I didn’t get and then tried to reset for normal to try again – a no go. The lockouts are indeed shared.
Small note, while it’s not the Heroic version, Protectors of the Endless has the “Elite” unlock that requires you to kill the bosses in a “harder” order. It’s possible the poster you’re responding to is confusing that with raid difficulty unlocks.
Just had a quick look, now with the level squish, the Elite version isn’t even different from the regular version so lol.