My 1st guess was a missing glyph on my fonts. So i updated my entire system (settings>text&fonts>fonts, adjust all fonts) and used a font from the Nerd group which is supposed to have bazillion glyphs. And I still get those blocks.
nobody complaining about this so I believe it might be something on my system only.
IIRC that’s caused by the font having a glyph for the NUL character. From the comment below, which I obviously didn’t follow through:
While playing around with different fonts, I noticed that some of them don’t like the ending ‘\0’ at the end of strings returned by xkbKeysymToUtf8, so I’ll probably create a MR for that.