If you are the raid leader, the raid will despawn bosses until it reaches the first boss you HAVEN’T killed.
So if you joined a raid at Paragons, killed it, then go back later, it will start you at Immerseus. Every boss will be present, the only difference is you will not get loot/be eligible for loot from Paragons.
If you want to create a “Garrosh Lockout” you must have someone who has killed ALL the bosses – without skipping ANY – up to Garrosh. This character now has a Garrosh Lockout. If that character wants to HOLD ONTO that lockout, they need to invite their raid, zone in, then transfer leadership and leave.
They will then continue to hold a lockout.
PSA – If you have a lockout like we did, where a toon had killed ALL of the raid except Paragons, we zoned in, at Paragons. That toon did not leave, but stayed for Paragons, she now has a lockout (despite extending the other one) that has her ONLY saved to Paragons. So if she tried to raid lead again, it would start at Immerseus.