一旦: temporarily stopping work and a once-you-leave condition
Compare temporarily stopping work because rain intensified with the condition that leaving through a door prevents returning before closing.
Work temporarily stopped because of rain
雨が強くなったので、作業を一旦中止した。 means that work was temporarily stopped because the rain intensified. 一旦, read いったん, qualifies the stopping of work; it does not report that the rain stopped.
作業 is what was stopped. The cause and the past stopping action are supplied, but no exact restart time or promise of resumption is stated.
The condition after leaving once
この門を一旦出ると、閉館まで戻れません。 says that once someone leaves through this door, they cannot return before closing. 一旦出ると links leaving once to the restriction that follows.
The stated boundary is 閉館まで. Do not turn the sentence into permission to return shortly after leaving or expand it into a permanent prohibition.
Keep the two time relations distinct
The first example reports a temporary work stoppage in the past. The second states a condition that applies after leaving through the door. Their shared 一旦 does not make their time relations identical.
Locate what was stopped and what action triggers a condition. A work restart schedule and the restriction on returning through the door are separate matters.
What differs between 一旦中止した and 一旦出ると in these examples?
Show the answer and why
The first reports temporarily stopping work because rain intensified. The second states that leaving once prevents return before closing. No end to the rain or exact work restart time is supplied.