Check the sidewall. If Nylon cord usually it says so. Nylon used to be used for a lot of tires but the automobile industry had dissatisfied customers due to the flat spotting mentioned herein. So the industry went to polyester and away went the flat spotting.
However my riding lawn mowers do not profess to have nylon cord but do sit for a long time. I run the air low, around 10 to 15 for a smooth ride. Every one of them flat spots and I have to run it out for a few minutes before rounds out.;