If vegetables are cooked submerged, what is the primary detractor causing loss of water-soluble compounds?

Immersion in the cooking liquid.

Water-soluble vitamins, exemplified by Vitamin C and the B vitamins, face two primary pathways to loss when vegetables are cooked in a pressure cooker: thermal destruction and leaching. While the high heat contributes to degradation, the single most significant reason for losing these compounds when vegetables are submerged is their diffusion into the surrounding water. When food is fully immersed, these compounds readily dissolve out of the vegetable tissue and into the liquid. If this cooking liquid is subsequently discarded, the nutrients go with it. This phenomenon explains why steaming—which avoids direct submersion—is superior for retaining these sensitive components, making the simple immersion aspect the main hurdle for nutrient preservation during conventional pressurized cooking.