I took the liberty of scanning back over some of your prior posts regarding this child, and I tend to agree with your interpretation that she is making careless errors because of instructional mismatch--aka, she is significantly underchallenged, and consequently struggling to find engagement or motivation in the tasks offered to her. I notice also that this has been a pattern stretching all the way back to her official kindergarten year.

It may be worth considering that pushing her to try harder on unstimulating work--while possibly having some value ultimately for lessons in delayed gratification--may not be addressing the core issue in this case, which appears to be that she doesn't perceive any mid-term or long-term benefit from gritting her teeth through what likely is highly unmotivating busy work. And this perception is arguably justified, since no past efforts to work more conscientiously on easy work have resulted in more interesting, challenging work. In fact, the one year in which she appears to have zoomed ahead with pleasure netted her a repeat year in lower-level work. (I am not blaming you, tw, for this at all; the schooling situation did not turn out as you were expecting, and you did the best you could with what you had to work with.) So one can see why logically she might see no reason for her to do any more than the median expectation on whatever is placed before her.

What you are describing sounds much more like instructional-underplacement-induced apathy than social masking to me.
...pronounced like the long vowel and first letter of the alphabet...