The LDML spec Collation Settings table says for the kk=normalization attribute: "If off, then all strings that are in [FCD] will sort correctly"
This is not quite true. If a string (FCD or not) contains one of the Tibetan precomposed vowels (U+0F73, U+0F75 or U+0F81), then the precomposed vowel must be decomposed or such a string might not sort correctly. The problem is that any contraction with the second part of the vowel decomposition needs to skip the first part. (Discontiguous contraction matching: UCA algorithm S2.1.1-S2.1.3) The DUCET itself has such contractions: The precomposed vowels’ decompositions themselves.
Suggestion: Change the normalization attribute spec to say "If off, then all strings that are in [FCD] and do not contain U+0F73 nor U+0F75 nor U+0F81 will sort correctly"