In KDE's "System Settings -> Icons" i have "Breeze" and "Breeze Dark" and if i select "Breeze Dark" then the icons are missing. Do you think this is an issue which should be fixed on the KDE side?
It's confirmed to be a KDE issue, although there is a mention of a potentially "unsolvable" issue (at least when it comes to their relative willingness to solve it), so you might want to check out the comments there and workaround it if able and/or want to.
I believe those buttons are all using standard GTK icons - perhaps you're missing the associated icon theme for that color scheme?