The most likely problem with your .018 patches is they are old, pre-lubricated patches.
The lube companies use to make pre-lubricated patches will slowly break down the cotton fibers in the material. After a year or more, this break down can cause the patches to rip, tare or burn thru like the patches in your photo shows.
Other things that can cause the problem is a sharp corner at the muzzle where the cone meets the rifling and the bore. All of the factory made guns have sharp edges at this location. The reason it can show up with thick patches and not with similar thin patches is, the material has to be squeezed more as it enters the bore. This additional compression can tare the cotton fibers on old pre-lubricated patches.
The answer to this is, if the muzzle has sharp corners where the crown meets the bore and rifling, remove them.
If the patch strength is in question, buy some un-lubricated patches, lubricate them with something ranging from spit to Stumpy's Moose Juice or Moose Snot or even Bore Butter and give them a try.
If you want to remove sharp edges at the muzzle, get a piece of 220 grit, black, wet/dry sandpaper. Tare off a 1" X 1" piece and place it on the muzzle.
Use your thumb to press it down into the bore and then start rotating your hand back and forth as you rotate the barrel.
Change the paper often and keep on doing this until all of the edges look like they are smooth and rounded off.
After you do this, a lot if not all of your problem, will go away.
