Answer
This is very likely a font issue, not a data/syntax problem — the font used for that field in the layout may not include the Unicode glyph you're trying to display, which typically renders as an empty box/square rather than the intended character (or nothing at all) when previewed. Use Report Designer's font-substitution feature to map the layout's current font to one that does support Unicode glyphs (for example, substituting a font like Garamond with a Unicode-capable font like Arial Unicode) — once mapped to a font that actually contains the glyph, the same UNISTR-embedded character renders correctly. Note IFS also ships a dedicated Unicode font as part of a separately-licensed UCFONT component, if a broader/more complete Unicode glyph set is needed beyond what a standard system font substitution provides.