Input:

birth\u{ad}day
birth­day // also contains a soft hyphen

In the resulting PDF both soft hyphens are searchable as spaces (birth day).
Expected result would be that the word is searchable as birthday (no soft hyphen included) or as birth­day (with soft hyphen included).
Including a space instead, changes the semantics of the text.

With LuaTeX, soft hyphens are not included in the searchable text.
Maybe related: #479