tr -s ' ' squeezes repeated spaces and misses most of what causes matching failures in real data.
The characters that break comparisons
U+00A0 non-breaking space is the usual culprit, arriving from HTML and PDFs. There are also en and em spaces, the zero-width space, and the byte-order mark, which frequently appears at the start of the first line of a UTF-8 file and makes the first value mysteriously unmatchable.
Regex whitespace classes vary
\s in a Unicode-aware engine matches the full whitespace category; in a byte-oriented one it matches only ASCII. [[:space:]] in POSIX tools depends on locale. Two implementations of "strip whitespace" can therefore disagree on the same input.
The reliable sequence
Normalise to NFC, replace every Unicode whitespace character with a plain space, collapse runs, then trim the ends. Doing this on ingest — rather than at the point of comparison — means everything downstream can assume clean input, which is a much easier property to reason about than remembering to normalise before each operation.
Try it: Normalize Whitespace on SeoWolf's Notepad.