Flexible raid lockout system is bugged as hell (got a few similar cases and a GM response confirming it).

Basically:

with a fresh ID, you kill or attempt a boss on hard mode, the next day you won't be able to set the raid to heroic unless you bring the exact same 25 players, as people get now saved to different hard mode id's; which only leaves you to clear the place on normal (even attempting a boss on hard mode is enough).

example: lets say that you want to attempt chimaeron hard mode, but you have no other bosses down in BWD:
smart thing might be to clear defense system, magmaw, maloriak & atramedes on normal, and come in the next day and spend the whole night on chimaeron progress. If you would 've done a hardmode attempt the previous night on him, and you bring a new person the next day, you won't be able to set it to hard mode.

This bug is at least active on several EU realms.

Not really boxing related, but might save some ppl from screwing up their progress for the week.