Okay: When was the last time that " TF" took that spring loaded Spout apart and cleaned the gate??? Powder residue coats those thin, flat-metal gates with great regularity, particularly on hot, humid days. It coats both front and back. The residue build up prevents the gate from closing properly, and that can allow a spark to reach the main charge in the horn. The ones with the round post, or tube, plunger take only a little bit longer to fail to close properly.
Like you, I also put the powder horn on my strong side( Left, in my case.) When I am shooting my LHed flintlocks, the locks then are on the other side of the stock from my body( and powder horn) and closer to the right side of my body than my left. When shooting a RH flintlock, I have to turn my body to insure that the flash powder does not set my shirt sleeve on fire.
I also make a habit of pushing the horn back so that the spout of the horn catches on the strap of my range bag, holding the horn back around my kidneys and behind the left side of my body. It takes no effort at all to reach back and lift the horn up and around the strap to bring it around to the front of my body to load the next charge in my powder measure.