Like most aspects in this sport, opinions and practices vary greatly and are based on the individuals results.... for their particular use. While most see little effect from mixing same lots, aside from the totally incredulous claim or two that, IMO, defy the laws of science, I expect you could practice this for a lifetime and never see any effect on results. This may not be the case with others. I have experienced mixed results. What I find counter-intuitive is that there are shooters that will go to extremes to insist on "same lot" brass, bullets, and primers, but, mix lots of the of the one component, the propellant, which has an order of magnitude in the sophistication and complexity of the production process, and a major influence on results. Whether due to liability reasons, performance reasons, or both, of all the components used to construct a cartridge, it is the "only" one that the manufacturers emphatically state that you should NEVER mix different lots of the same powder....because of "burn rate" variation in the lot specs Burn rate plays a key role in a variety of shooting factors, precision, ES, temperature stability, and pressure with the finished cartridge. Producing the propellant involves the incoming testing of numerous raw materials, processing steps, and test procedures throughout the entire process. Evidently, the current "State of the Art" has not yet reached the desired level of lot to lot consistency.
While I have certainly mixed my share of same lot powders over the years, since taking up serious LR hunting and competition, I no longer mix. Whether there is a material difference in my results could certainly be debated, I'm more then happy with the results, and the trade-off of not mixing seems irrelevant in the grand scheme of things. IMO.