Had a similar experience. I gave a low but passing grade to a student. She'd missed my conversation class to practice for and attend a speech competition. She took first place. It was the national competition.
I had forgotten she'd asked for an excused absence. I was really busy at the time and looked at my grades and averaged them and submitted them.
So I realized I'd made a mistake. And I realized I'd given a low grade to the national champion. So I asked to change her grade in the system, but only after she'd made her missing work.
Yeah, I was penalized and lost part of my bonus. For wanting to set the record straight. Well, I paid the price, because it was the right thing to do. Go figure.