I would expect to see reflections in the unterminated stub, since, as seen from the stub there is a piece of 50 Ohm coax terminated in 25 Ohms. Reflections from the unterminated end would not be absorbed by the source (as seen from the stub). So you have both an impedance mismatch at the source and at the load (unterminated stub). Attached is a quick and dirty LTspice simulation (blue is source, red is terminated load, green is unterminated load). Obviously a shorter stub would shift the standing waves to higher frequencies, but not even in the ballpark for ppm-level performance.