Classic Legacy raids and lock outs

I recently started mount hunting again and noticed the issue that didn’t occur to me before: I now can finish raids from older expansions on one difficulty only. With more recent expansions it works just fine: I can clear Antorus up to dog bosses on every difficulty, it only takes to go out, change difficulty to the next one and go back in.However it didn’t work for Pandaria and WotLK ones. I cleared Mogu'shan Vaults on normal, then got back and changed to HC but it still takes me to already cleared NM like if I am locked from all difficulties.Is it how it supposed to be? Or am I doing something wrong? Have you ever had the same problem? via /r/wow https://ift.tt/wTmHfSW