Stainless steel is stain-LESS not stain free. It will rust and the way it rusts can be worse then carbon steel rust.
In order to prevent rust you shouldn't just fire the rifle and then put it aside and not expect it to rust.
The old timers who said "Never let the sun set on an uncleaned gun" were right then and still largely right today.
You CAN get away with storing an uncleaned and unprotected gun, but then you will get caught short once in a while.
As above, there could be something "off" about the barrel steel. I'd contact the maker and ask them what the Hell!.
To prevent rusting in a gun barrel all that's really needed is to do at least a partial clean to remove carbon and powder fouling, then run a patch with a few drops of CLP Breakfree through.
CLP is an excellent rust preventing product and I've never seen rust on a gun that had an intact layer of CLP on it.
There are plenty of other products, some people just leave a film of Hoppe's #9 bore solvent in the barrel.
This prevents rust and works on removing any copper or carbon fouling.
Also, much depends on where and how you store the gun. Put it in a damp place like a closet or basement and there's a good chance it'll rust.
If you have a situation where it's simply impossible to do even a quick clean and application of as rust preventing lube, if nothing else spray a good shot of a spray lube or even WD-40 down the bore and store it muzzle down so the excess will drain.
Almost ANYTHING is better then nothing.