I do recall this "invisibility myth" from my youth. However, I never found much to support it. Only one thing, but I cannot find it now. It was that the mossie's signal was weaker than of a typical aircraft it's size, thereby giving it a fake ID.
What we DO know however, is that tracking a fast target is harder than a slow one, which could have generated a problem for the German tracking system (Wurzburg would be aimed at the target, being guded there by the Freyja system before)
What we also DO know is that NF mossies were used for both bluff and jamming missions, spreading "window", - i.e. a metal foil.
The biggest bluff of all times would still be the one made by the Lancasters og 617 sqn. At the night before the Normandy landings they created a "ghost fleet" heading for pas-de-Calais simply by dropping window in the precise time and locations.
By the way, - Mossie pathfinders had the lowest loss rate of any Bomber command type, - 0.03%