The bug
I generated a buffet world with the swamp biome selected. On both swamp and swamp hills, strongholds don't generate. I used /locate
only to find nothing. If this is your intention, probably to make strongholds not cut into ocean biomes, I should note that they generate just fine in stone shore biomes, which generate adjacent to oceans.
From quick code analysis, SwampBiome.java does not contain the addStructure for Strongholds, only Mineshafts and Witch huts, so it might be intended