Do all tundra portals (used portal energy) all lead to another island with tundra portals? I came across 2 islands (level 2, level 6) without any other portals to continue. Is this a bug or is this made on purpose? 

There's always a chance to run into a dead end. The bigger the island, the more chances it has another portal leading deeper into Tundra. We recommend using high level pets to explore Tundra when the purpose is to reach deeper levels.