I poked around. Her percentile is 89th, down from 98th in 3rd grade (assuming both were winter tests). That is nearly a standard deviation difference, so approaching a significant stall out, though not unheard of, as the grade 3 test top items would be limited in comprehension depth, while the grade 6 top items could be substantially more abstract inferential comprehension questions; the ceiling on grade 3 reading is likely lower than that on grade 6 reading (even though it's adaptive, it's limited by the level of the start point, and the number of items). I do also see in the norms documents, though, that in addition to overall RIT scores falling from the 2011 to 2015 norms, the standard deviations also rose. So a number of qualities of the norms changed, which may make it more difficult for schools to interpret these numbers clearly.
I think my main question would be whether the school may be able to administer a more in-depth measure of reading.
