Stains Inside Washer Drum

Hello,

In February I bought a Whirlpool WTW6120HW3 washing machine, and I notice inside the washer drum a stain ring appears in the drum (see images). These stains do not go away with normal washing, but rather I need to use scrub using a microfiber cloth with a bit of detergent. Unfortunately, after the washer goes through just one more load the stain reappears.

I contacted whirlpool about this and they advised for me to use 2 Affresh cleaning tablets rather than just one when cleaning the machine, however, this does not remove the stain. I even tried removing the stain with the microfiber towel then using Affresh to see if it would keep the washer drum stain-free for longer, but alas this is not the case. Our old washing machine never had this issue, and we do not have hard water as other utilities such as our toilets do not get stained. Any help on what I could do would be greatly appreciated :)