With todays rifles, normally a frizzen is hardened thru & thru, not a surface hardening. Some guys do the Kasenite thing but I have never found a reason to do it. IMHO your old frizzen was most likely not hardened well in the beginning or it was not the same geometry as the new one, as a very slight difference can make a world of change.
Sometimes on old rifles you see a frizzen shoe (new face) sweated onto a frizzen. Most of the time this was not to give it a harder face, but rather to replace the removed metal from thousands of firings, thus putting back the correct angle of the face of the frizzen to make the lock perform properly.
I have a Lyman GPR with over 5000 shots from the same lock & it is all original. It looks like H but it still fires each time & I will continue to use it til it dies, then I will buy a new frizzen for it.
One thing that will help the life of your frizzen is to reposition it about every 20 shots & use the entire face of the frizzen by moving the flint back & forth so it hits different places on the frizzen. If the flint gets to tracking in one place all the time, you get less sparks & all the wear in one area.
Getting back to the original post & question....... I have left my ML loaded for a week at a time with no ignition problems (years ago) but now I try to shoot it every evening after the hunt. I have left it loaded 3,4,5,6,7,days & it has Always went off when shot, it is just that I feel I need all the practice I can get off hand, so when I come in from hunting at dark, I turn on the spotlight at the range & shoot at a small spinning target. Or I may take a shot at the 100 yard dinger plate just for fun, but I like to shoot the rifle at least one time a day...... If I am hunting someplace where I don't have a target, I just pick a leaf on a bank or something before dark when I can still see..... At that late it is too dark for me to make a sure kill shot on a deer with open sights anyway & I dang sure don't want to track one at night, so I would rather work on the marksmanship.
Birddog6
Custom Muzzleloaders & Custom Skinning Knives