The following would have been a support request had I not figured it out. But I am sharing my experience in case it helps anybody and possibly to suggest that a note be made in the set up guide about this.
I was just going through the process of importing music onto an empty SSD drive that I installed in my K21. My entire library is stored on my Synology NAS, and using my Mac, I imported all of my music using the ‘Upload’ feature in the web interface. However, this results in a certain encoding that causes any file or folder name with accented or certain special characters to become not visible and appear to be missing when accessing the K21 through the Finder via SMB.
What I ended up doing to solve this was to start over again and import music by copying the files with Finder directly over SMB and foregoing the web interface altogether. This solved the issue completely.