When creating a superflat world with a set biome (in this case Plains), occasionally Desert Temples and Jungle Temples will spawn despite not being in the right biome. I have provided two seeds and coordinates where this has happened. I got lucky with the Desert Temple since it spawned right next to a village. Some crops in a village farm were planted on the sandstone which gives me the impression that the temples spawn after everything else, as it overwrote part of the village.

Superflat Preset I: "Overworld" minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block;1;village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon

Seed I: -6218231414295295201

Desert Temple in Seed I: Near 109,181

Superflat Preset II: "Overworld" (modified)
minecraft:bedrock,59*minecraft:stone;1;village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon

Seed II: 1234

Jungle Temple in Seed II: Near -963,-227