Sounds like another random bug. Welp, add it to the list of glitches that will no doubt be difficult to fix. Most of the time, an island being part of the same continent doesn't prevent treasure fleets from spawning as long as the treasure resources are there. Usually about 50% of the islands are part of the same two continents as the starting landmass (assuming you're playing continents plus). They tend to be evenly split among the four continents in the game.