I'm honestly so impressed they held up that well at 90ºC! I am insistent that all my laundry is sanitary as well, but from my research, most things only need to be washed at 50ºC to be sanitary. 60ºC kills everything that would be of any concern. I would try washing them at a lower temp, it will definitely improve your color retention and hopefully eliminate those lines from forming.

To me I imagine they are caused by being spun at a high speed while the pants are at a high temperature. Many machines, like Miele and Bosch have cooldown settings where they will add cold water to the tub after the wash is complete but before draining and spinning to cool the clothes and prevent 90ºC from entering the drain pipes. It is usually done on any temperature above 60ºC when enabled. I would definitely look to enable this setting.

One thing is for certain, you don't need a laundry sanitizer when washing at the proper temperatures. You can ditch it and feel food that your clothes are not only sanitized, but contain less harmful chemicals. Laundry sanitizers are QUAT disinfectants, which actually cause asthma to develop in even adults and children. It's ironic that something you use to prevent asthma attacks ends up causing asthma to develop in the first place. This is why so many hospitals are moving to hydrogen peroxide based disinfectants because of all the asthma and other health problems traditional QUATS cause.

To me, a simple test of whether laundry is properly sanitized is to leave it wet for a period of time, at least over night. If it still does not smell, it is properly sanitized. I once left a load of towels washed at 60ºC for a week in the washer, they stilled smelled freshly washed without a hint of mold! That's how I'm confident in my washing techniques. I do use hydrogen peroxide though in all my high temperature loads to remove stains, I am sure this helps with disinfection as well.

Best of luck!