@jresoull The door does not open based on character presence. There are a few things that can cause this such as getting to the door too late, not exploding the core, joining a server with 'The Core' already spawned in, etc.
It doesn't seem to be a bug, hope this clears it up! ^_^
Im making a second account so that I can post non-professional stuff or be more social with.
Consider following if you want to see random stuff of mine more often :o
(may not always roblox related!)