There has always been a running debate as to whether Invasion maps should expand or progress - whether the map should grow in size and retain the original play area, or move across the canvas and effectively play the Round across multiple maps. But the fact is that the map is simply one map and does not expand nor progress. Instead, the Game Experience grows.
Tier 2 is not accessible during Tier 1, because the players are isolated in Tier 1 until Tier 1 is won. This is to prevent the Invaders from moving into position to capture a Tier 2 Objective as soon as Tier 1 is won. Such a move would be unfair to the Defenders who are not afforded the time to setup a defense. The point is that the isolation of Tier 1 is not indicative of a separate map, but rather of game flow control forged into the map.
All three Tiers play differently on the same map. This means that while Tier 2 and Tier 3 generally play in the same section of the map, they do not play the same - because the Tiers play differently from each other, but on the same map.
Tier 3 Objective (the Core Capture Zone) is present on the map during both Tier 1 and Tier 2, but it isn't used during Tier 1 and Tier 2, because it is only used by Tier 3. It has to be present on the map to be a believable part of the map when Tier 3 begins.
Tier 1 is playable during Tier 2 and Tier 3, because it is still part of the one same map and there is no purpose to isolate it. In fact, a wise use of the Tier 1 play space during Tier 2 and Tier 3 is to spawn Invader vehicles, using the sheer distance as a natural obstacle against Defenders from jacking the vehicles from their spawns.
The map is one map and it helps the forger to see the map in this light. Every aspect of forging Invasion, the different Game Play, the different use of Tier 1 for vehicle spawns, the Core Capture structure, all make sense when the map is viewed as just one map for which three different Tiers are played on.
The map is just one map with game flow management forged into it.
The Invaders must not be permitted to enter Tier 2 until Tier 1 is won to maintain fair play.
One can wisely use the sheer distances to discourage jacking of Invaders' vehicles from their Spawns.