My question is...Is it a concern that she is more than capable of devouring chapter books geared toward first and second graders but still isn't?
You have described my son who is now 7. He perfectly fits the profile that you shared of your daughter, except he went through a period around 4 where he wouldn't read to us at all. I just let it go and didn't push it. Then, somewhere around five or so, I began to catch him reading more advanced material... ahem, excuse me... "looking at" more advanced reading material. That seems to be what he wanted me to think, that he was just looking at the books.
He loved being read to, and we did that often, but he also sat and read in his book closet on-and-off throughout the day. I knew he could read. I just didn't know how well he could read at this point. One day I walked into the living room and caught him reading the newspaper. I asked, "What are you reading?" He replied and then he realized that he had ratted on himself. I just grinned at him and he sheepishly grinned back. Then he ran away and hid in his book closet. LOL
I decided at that point that I would just get him to read a sentence here and there. He really did not like doing this, as he seemed embarrassed or something. He did well, so I persisted, but very gently. I would get him to tell me what short sentences on boxes read, things like that to coax him out of his shell. Finally, just a short while later, I got him to start buddy reading with me at night. He still much preferred that I read to him, but I kept at it and slowly increased what I asked him to read.
Once he started schooling (he's homeschooled) there was the expectation that he would read much more. It really didn't take long before he just gave up and whatever had been preventing him from reading aloud to us simply went away. He was immediately reading through books like "Charlotte's Web" and "Stuart Little" in kindergarten. As I exposed him to harder books in first and second grades (junior high level books in 1st grade and high school level books in 2nd grade), he stepped right up without any issues, though he did have to get used to the older style language of some of the books that we read. He does still read some books on lower levels, but he clearly prefers the more advanced literature now. I suspect your daughter will be the same, esp. with scores like that. I would be gentle with her, but I would also stretch her at times, too. I'll bet that she'll rise to the occasion in the next year or two, if given the opportunity.
