Hello Donald, 

Wow, this is a mystery! Just to double check, have you looked for grubs? You can confirm by pulling up the section of grass there to see if there is anything underneath. I know you mentioned replacing it in the past, but it might be worth another look to rule out any grub damage or other worms. 

Another option could be to send a sample to the UMN Plant Disease Clinic to confirm there are no diseases, although it would be unique for the disease to occur only in the 12 inch spot in your lawn: https://pdc.umn.edu/

Finally, have you tried reseeding the lawn? I realize you replaced it with a growing spot, but a quick reseeding in late summer might just be enough to cure the spot. Here's a helpful UMN Extension article with more tips: https://extension.umn.edu/lawn-care/seeding-and-sodding-home-lawns

I hope this helps!