Possible bug in utrie_enum():
This seems to be left over from the initial compaction which only overlapped adjacent data blocks if they have the same values in the adjacent parts. Later, compaction was improved by looking for whole-block matches in previous data, which can fully overlap adjacent blocks if they contain the same data – but not necessarily the same value throughout the blocks.
If this is indeed a bug, then there is a potential for UnicodeSet to be wrong for properties with unfortunate data distribution.
Investigate, confirm, and fix if necessary.