I don't know, but I'll be watching this space carefully-- any chance that she also has perfectionism issues in play? That has led my DD to have wicked underperformance on exams-- and while most of the time it isn't terribly noticeable, it's enough to drop her a full grade in STEM coursework at the college level, and it shaves off a few percentiles here and there in other assessment, too (So national merit commendation, not semi-finalist, that kind of thing). She does MUCH better on essay exams where she gets deep into the test and isn't watching the clock or doing a lot of shallow attention 'task-switching,' where she feeds her anxiety.
My DD had a somewhat similar pattern with math-- it really wasn't until calculus that things didn't look so rosy. Honestly, one problem was that she simply wasn't accustomed to NEEDING to work practice problems, and so she secretly feared that there was something wrong with her for needing to do her homework.
I also think that there has been more of a tendency to focus on THE right answer, as opposed to (in years, er-- maybe decades past) an emphasis on a correct APPROACH, which, if arithmetic were correct, would lead to a numerically correct answer. I mean, sure-- some point deductions for sign errors and bonehead mistakes were taken, but it wasn't so all-or-nothing. It's more labor intensive to grade that way, I suppose. It's why homework is no longer graded, either, in most instances.
I'm sure that you've thought about a stealth LD already. Just the same, I'd look at the anxiety piece of things first.
