creamatorium level 2 already cleared every time I enter with a party

This is a walk through break bug.

It should be your highest priority to fix this.

Repro steps:
1) Join/start party in Act 3
2) Enter crematorium for quest
3) Get to crematorium level 2
4) It's already cleared, preventing the player from finishing their quest

This isn't really a bug. You can either solo the area or create a new instance and clear it by holding control and clicking on the door.

If the party you joined has already cleared the area then that is normal. Now if you are stepping into an instance that is not related to your party that is a little different.