Consider this program,
sql = [sql| SELECT f1, f2 FROM aTable |]
xml = [xml| <html><body id="hi">hello</body></html> |]
I have ("sql" . sql-mode)) in haskell-font-lock-quasi-quote-modes yet the body of the quoter gets no syntax highlighting whatsoever. If I past the body into a new buffer and enter sql-mode manually it is highlighted as one would expect, so presumably it works as expected. The xml quote is also highlighted as one would expect, so quasi-quoting highlighting isn't generally broken. It seems to be particular to SQL.