Help with stained MDF cabinets

My house has MDF cabinets doors and ends. They’re stained and we’re built about 7 years ago. I’ve never dealt with stained MDF before. This is on one of my cabinet ends and I’m not sure how it got there. Maybe bleach or something else splashed up?I’ve tried to rub some of the “touch up” stain I was given but it does nothing and just wipes off. Any suggestions on how to fix this shirt of sanding it down and restaining the entire piece?

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