One I can think of as a weak reason is Averting the designated hero trope( link )
What some people might've felt wasn't a character flaw, but a problem people have with a character intended to be heroic, the problem being that the hero did something considered a kick the dog moment.
A good example of how their argument might be weak would be Aly from Tortall Universe, who qualified as both a Designated Hero and a Mary Sue, see here: link

What's more, if they think the jerkass trope( link ) can prevent them from being a mary sue/marty stu, I disagree because that doesn't always work and can lead them being to a jerk sue( link ), which might make things worse.