I'm sure you and all of us know the answer to this. A lot of readers for this genre are women and as such, they tend to be harder on women than men. In some cases, its the dreaded internalized misogyny, in others its a case of being disappointed in someone of their gender being like that because they believe THEY would never be like that and they expect more from women than men.
Another reason is lower expectations or 'gap moe' if you're familiar with the term. MLs are portrayed to be cold, aloof, rude, violent, arrogant, etc. So when they show ANY redeeming quality or say one thing nice, it has more impact. Its like when FLs who are expected to be pure, devoted, friendly and people pleasing show any flaw at all, they get ripped apart.
The last reason is very shallow and the saddest of all: hot guys get a free pass. Theres manga where hot guys just kill people for no reason, they just like killing, no revenge plot, nothing. Theres manga where hot guys are actually abusive, overbearing, CRIMINALS toward the FL but they're hot. Anything a hot guy can do is fine because they are hot. The readers are livid when a FL does any of that shit because how DARE they when they're not even a hot guy?
That being said, these are my general observations after reading manga (and the like) for over a decade), I haven't personally seen any on this one in particular.
you are right, and sadly.. its the same readers who feel that way that also say all isekai / reincarnation stories are practicaly the same..
Yet hate every time a new type comes out and says stuff like "why didnt fl do what that fl did"... and in 70% of those stories they refer to FL got out of debt / a sticky situation by marrying a rich nobel.. aka they sold themself but its all good and dandy because the huband comes to live FL in the end..
And in the last 30% Fl is from a high ranking nobel family.. aka money and power.
In this story FL is a commoner with no father and isnt considerd "pretty" and is stronger and more talented than most nobel men.. hence the nobel men feeling inferior to a "common girl" and hence all that bad rumors about Fl.. Think about it .. the bad rumors were there befor FL literaly sold herself to the black knight..
ok.. let me get this straight.. a manga with a FL who worked and did anything for money to pay off the debt from having to care for her sick and now dead mother is a scumbag..
But why dont i hear the same badmouthing and critisism of mangas with a Ml who does the exact same? In the mangas with a ML who was like this.. ppl validate it as him "doing what he had to do to survive" and aplauding the "changes" he makes when he falls for FL...
Yet here we have a FL experiencing the same yet still being judged even when she makes changer... and the argument is "she was shit so she got what she deserves" and "she should abandoning everything and starting anew in a new area".. SHE HAS NO MONEY.. how would she start anew with no cash and debtcolector hunting her? Honestly..
No one questiones a male character who has been trained and raised all his life to be a knight to pay of a debt..
So why are so many quetioninng the FL about the same..
How many stories have we seen with Tyrants ending up with a Fl and ppl going "he changed for her" .. and yet here we are and FL somehow deserved worse than death?
I am literaly not hearing much critique of FL's ex comander who orderd here to do all that bad shit.. and im even seeing ppl blody defending him and saying he is better than her.