Missing graphic elements in compose window

[Case_Of]

Hello,
I wrote an ebuild to install NaSC on my Gentoo system and try it.
I can see some graphic are missing. I do not know if it is caused by a missing theme or font.
In the example attached, ans is missing but appears in exported PDF document.

PDF document exported
(mirror)

[Dave Webb]

I also get this on Ubuntu Disco when compiling from source with 3452e9d

[Peter]

I can't reproduce this on the elementary theme. So maybe it is a theme issue? Anyway I need more info to reproduce this.