The phenomenon you're describing, where a particle and its corresponding antiparticle spontaneously appear and then annihilate, is known as particle-antiparticle pair production and annihilation. It is indeed a process that involves the conversion of energy. However, it does not violate conservation of energy because the total energy before and after the process remains constant.
In particle-antiparticle pair production, energy is transformed into mass according to Einstein's famous equation, E = mc², which states that energy (E) is equivalent to mass (m) times the speed of light squared (c²). When the energy available is sufficient, it can be converted into the mass of the particle-antiparticle pair. This process obeys the conservation of energy because the energy before pair production, which may exist in the form of other particles, radiation, or an external field, is transformed into the mass of the pair.
Once the particle and antiparticle are created, they can quickly annihilate each other, resulting in the conversion of their mass back into energy. This annihilation process typically produces two or more photons (particles of light) that carry away the released energy. The photons can have different energies depending on the initial mass of the particle-antiparticle pair and any additional energy present.
So, while the appearance and subsequent annihilation of a particle-antiparticle pair involve the transformation of energy into mass and then back into energy, the conservation of energy is maintained throughout the process. The net energy change is zero because the total energy of the system remains constant, considering both the original energy sources and the resulting energy in the form of photons or other particles.