I'm also seeing that issue here using Ubuntu 20.04 (Adwaita icon theme) and Kubuntu 20.04 (Breeze icon theme). When switching to the Yaru icon theme fully legible icons are displayed in Ubuntu.
A quick test, which might not be totally accurate, shows me that only by selecting Yaru are these icons displayed correctly.
I'm also seeing that issue here using Ubuntu 20.04 (Adwaita icon theme) and Kubuntu 20.04 (Breeze icon theme). When switching to the Yaru icon theme fully legible icons are displayed in Ubuntu.
A quick test, which might not be totally accurate, shows me that only by selecting Yaru are these icons displayed correctly.