OnlyMe, can you give a link, where mass loss is revealed as you stand for?
Emil, within thermonuclear reactions I include normal radioactive decay, which may include neutron radiation and EM radiation under normal conditions and a variety of broken atomic parts in a bomb. The only aspect that involves a loss of mass if you account for ejected particles as conserved mass, is the EM radiation. For that the easiest reference is the first, I know of on record.
DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?, Einstein's 1905 paper that introduced the equation E = mc^2 (though in that paper he used "L" in place of "E". E was later used in the formula to standardize terminology.)
The paper essentially says that when an electron in an atom moves between energy states it emits or absorbs a photon and that in the process the atom loses or gains mass, equal to the energy of the photon divided by c^2.
Photons extend in wavelength far above and below the visible light spectrum we associate with light. At least a portion of the heat radiated away from any body is in the form of heat in the EM spectrum.
If you accept the equation E = mc^2 and Eistein's paper introducing it as valid, then when any process involving mass or an object, radiates heats in the EM spectrum it must also lose mass. Which in classical terms, weights and measures, is insignificant, but cannot be ignored.
While chemical reactions do not involve the loss of particles in the same way that a nuclear decay might, they do involve heat radiated at least partially in the EM spectrum and so, by the same logic must involve some transfer of mass from one atom to another, and a loss of mass where that EM radiation fails to be re-absorbed by another atom.