MusicXML import does not honour accidentals
You can reproduce this easily by creating an XML with a series of the same note but with different enharmonic spellings (e.g. F# and Gb). When imported into REAPER, they are all rendered the same way.
This is particularly confusing in notation view when an uncommon accidental is the one that makes harmonic sense, but rendered incorrectly (e.g. B# as the leading tone in the key of C# minor, but rendered as C natural). Since MusicXML contains this accidental information, and REAPER supports it too, it should be preserved upon import.