The google page seems not to support the '+' glyph, which at first seemed like a bit of a failing in a programming font - but then I remembered how many bugs string concatenation was responsible for and I realized they might be onto something.

Sadly, the version on https://www.recursive.design/ seems to include the symbol.

(spoiler: it's nothing to do with the font - it looks like it's actually a URL encoding glitch with the way the page manages the preview text - it can't round-trip a + symbol correctly)