It seems rendering of ref tags on aeroway=runway is slightly broken. Runways always have two identifiers (one for each direction) and these are typically written together in the ref tag separated by a semicolon. It appears this semicolon is being replaced by a linefeed character, which makes sense for the current highway shield rendering style. But on runways, the value of the ref tag is still drawn along the way, and multiline text along a path doesn’t seem to work, so instead we get a placeholder glyph that looks like a tiny “LF” in a box.

I think if we could get that semicolon replaced by an em dash (“—”) instead (only for aeroway=runway of course) we’d have a satisfactory resolution.