What I why strange is just leaving a key in the switch causes this. The key is insulated from the electrical system. Strange things do happen but this is very strange.
I, too, have a Craftsman rider mower purchased in 2016 that started doing that after 2 or 3 seasons. I purchased a new battery just like you and still had the same problem in a couple of months. (BTW,, I never left the key in the ignition.) I ended up getting a Battery Tender with a quick connect ring connector. Now I just plug it in whenever it's not in use. I never had that problem with the previous Craftsman I had for 12 years.
Craftman riders are now junk. In the first couple of seasons with light use with my current Craftsman, I had to replace a fan belt as well as a tire. Just last year, I had to replace the ignition switch because the reverse mow function broke. Even after 12 years, I never had to replace any of that on my old Craftsman. There's plastic where metal obviously needs to be. It built to have the shortest service life possible. When I have to replace my current Craftsman ( I seriously doubt it will last anywhere close to 12 years) I certainly won't be purchasing another Craftsman, that's for darn sure. Craftsman use to mean true quality but no longer.
It is because the consumer wants to buy the cheapest thing out there so even well recognized names have to cheaper products to keep prices lower. I haven't seen this in the commercial line up yet be it will even migrate to that area to. This because they are figuring out how to just get things to last through the warranty period so the customer has to upgrade again keeping the factory running.
I just ran into some Gravely (Ariens) spindles that are nearly impossible to get apart. Mainly due to rust. It doesn't help that the customer crossed threaded the spindle bolts. So it is two new spindles if he want them repaired. Ouch at 250 each OEM. Even the aftermarket ones are 150 each. He said he would run them until complete failure of the spindles. What a shame as the mower is well built otherwise. In this case I cant get new spindle so it a waste of time to try to rebuild them.
And yes it seems no one makes a quality product any more. It like the HFT torques wrenches the first ones I brought lasted five years but now I can't even get one that the scales align properly and the last two fail after a year of use. Junk with a life time warranty is still junk. Personally I depend on my tools working correctly. Heck I got a lot invested in them.
So I trying one of those digital torque adapters. I just don't like the feel, just not same as the clicker type. They are only as accurate as the user pays attention to the beeper. Maybe in time I get use to it but it will do until the new clicker comes in. Hopefully the vendor didn't lie about the new clicker being able to do both left and right torques. (CCW and CW)
Even things like wrenches are not the quality they once were. I even got a couple sets of Vermont American taps and dies where the lazer etched sizes are so fade off it is impossible to read them. Their lifetime warranty is worthless as their are no dealers for them anymore around here.