Per the mw.org manual cl_sortkey_prefix is supposed to be "the human readable version of cl_sortkey" (when the dafault sort key is not used).
For very long non-latin sort keys (e.g. قصر البارون امبان بمصر الجديدة.jpg) the sortkey (for Category:Cultural heritage monuments in Egypt with known IDs) gets cropped to fit in the table, however it appears that this cropping does not respect the encoding of the string meaning the cropping stops mid unicode character. As a result the cl_sortkey_prefix cannot be converted back to unicode and cannot be said to be human readable.
The desired result would be for the crop to be encoding aware and drop that last partial cahracter.