You're confusing TDR with overall Return Loss. The impedance magnitude of a discontinuity (via, pad launch, etc) is directly proportional to the edge-rate, as faster dv/dt equates to higher harmonic content. If you have split-plane in the return path on a common-mode signal (single-ended) whether it's fast or slow, there will be a substantial impedance bump which will not only impact the Return Loss (Reflections) but will also generate XTALK and EMI. The bigger issue is the generation of XTALK and EMI over a void. Especially in a densely routed parallel bus.