I'm just getting into this reloading game, and I certainly ain't no expert but, wives tale or no, what I do know is, The Hornady Reloading Manual, 8th Edition, Page 74, item 11 states...
Quote

on't use reduced loads of slow burning powders. Reduced loads can cause both high pressures and unpredictable results. Don't test your luck on the subject.
There's also a very
informative debate on the 1911 forum concerning reduced loads and this phenomenon, whether it be Flashover or Secondary Explosive Effect, whether it exists at all, or whether it be wives tale. And, while several very knowledgeable folks there suggest, much like some of you guys, that it doesn't exist, several folks there do reiterate the warning of reduced loads of slow burning powders in rifles.
And, if you Google "
Reloading Case Filler ", you will find the same argument all over the internet. Which... that's the internet where opinions are like, well you know.
I do know that I have found what appear to be credible arguments on both sides of this one, and then when you top that with the above cited reference in the "Don't" do this list, in the Hornady Manual, it tends to lend some credence.
The question was asked why folks pack their loads with cotton, and whether Flash Over exists or not, a large part of the reason they pack them is to prevent Flash Over. And, to keep all the powder down by the flash hole.