It doesn't. Conservation of momentum, which is an equivalent of Newton's third law, exists in General Relativity. The results of how a system would evolve under

It doesn't. Conservation of momentum, which is an equivalent of Newton's third law, exists in General Relativity. The results of how a system would evolve under GR and the special case like the one above just happen to look strange on a superficial look. If one makes the effort to calculate all the constituents of momentum in such a system, from GR equations, they d verify their sum is conserved.