Will having a joint between concrete beams weaken a structure?
Yes, it is weakened. The allowable shear stress at a cold joint is greatly reduced from monolithically poured concrete. However, concrete is not that great in shear strength anyway, the reinforcing helps greatly in supporting the beam end. Is the rebar and reduced bond strength enough? Only the beam designer can say for sure. I don't like it, the beam end is the very worst place for a cold joint, but I'm not the designer. It may be OK though not ideal, or it may be a huge problem.