There are two kinds of characters(usually,not always but sake of argument) flat characters and round characters.
This trope is that you have a round character that is too round
and have personality that is unnecessary complicated and extreme.
(Lol)This sounds subjective and you ask why is this a problem?
Imagine that you play a rpg, you are building a character from
a list of personality trait in the corebook.
Traits that everyone wants costs points and less desirable traits
give you points.
So the book divide all traits into polar opposites and neutral "quirks" are ignored.
You find some "extreme" personality traits that give points for compensating for being so hard to keep in-character.
You want a lot of personality traits to make your character cool and you also want as many of those cursed extreme personality traits as possible.
Perhaps you create ;"Bob B Snut
" who is Ridiculous [cultivated as hell] and analytical[stoic] by day but have a split personalities that he talks to that somehow chaotic[Dr Jakyl & Mr Hydes] but both sides coexist at the same time and so he can have the benefits from both opposite personalities at the same time."
Bob N Snut
has the trait "murderously compulsive-impulsive" and the trait "emotionless vulcan" at the same time so Bob B Snuts
should be plus/ minus = zero and a normal guy with average stability and aggression level which warrants no traits and no points. Though thanks to the impossible convenient split personality Bob now pool all the points and all benefits and he probably never keep in character but he sure got a great [mary sue].
Now transition this into the terms of a writer and remember there nothing to hold you back.
There are two great signs of this trope being active;
1. the writer clearly do not want the character to be normal minded person with no justification in the story or setting.
Example: "serial-killer baby sitting" becouse it's more interesting than the baby sitter.
2. the writer clearly do want his character to be able to do all everything a normal person does to advance his story.
the writer clearly does not his character to suffer any drawbacks or weaknessess,
It is like a berserk rage without control issues
or a hallucinations without nervious breakdowns
and post-traumatic stress without berzerk buttons.
The writer wants the bonus points of the flaw(making the character interesting) but avoids the difficulity of writing such a story or just wants the benefits of normality combined with the inner angst of the opposite flaw.
4.Nothing of this ever turns out plus/minus = zero.
Good actions never cancel evil acts, nor vice versa.
Behind all that self control and high morality the serial killer always have an insatiable rage and therefore he has no selfcontrol according to the writer.
The character doesn't undergo a tranformation and there isn't any real alter ego development that limits him from accessing his incompatible traits at the same time.
"A particular small wound is forever highlighted in the other wise perfect face even behind a mask", so give me my disfigurement bonus points!"
5. the writer will do not make choices. Status Que is good but for character development.
For example the writer want both the story of future earth ruled by a megacorp and the story of space feudal Emperor
so why not have both? A space Emperor with feudal megacorps!
Now we can have "robocop + star wars!".
(.... becouse it is a really.. really bad idea!).
The character do not make choices that excludes anything nor mean anything more. Everything the character experience is just a metonymy of the same experiences.
6. The traits are very selective when taking effect.
When the story needs to end, exept all the characters logical weakness to awaken from slumber and serve as an Achilles heel or Deus Ex Machina, and never before.
7. This is not really what it seems.
The character only appears to be normal or only appears to be good but is really evil anyway. In order for the writer to justify his premise he retreat to this kind of un-logic.
7. Not always but often a character suffers from this becouse writer is trying to create a character is very good or very evil
by simply adding a great amount of mutually exclusive personality traits and pretending to compromise them.
Remember, the stoic caped superhero who have no evil emotions but have the heart the size of a solar system and is openly heartwarming compassionate for all plants and creatures in the universe.
Least, the most interesting symptom.
Despite seeming to aim for interesting characters the writer
never seem to give any room for natural character development to happen after the story.
The character is so rigidly defined from the start that he doesnt leave much to imagination.
Protagonists can suffer from this trope often becouse
the writer wants them to be impossible sympathetic
or have an inner conflict.
Though mostly it is a case of the idealised compromise gone wrong.
Antagonists can suffer from this becouse the villain writes the story which motivates the writer to him arbitrary.
It could also be the case that the antagonist is a substitude
for many different people or groups