Some rendering artifacts in dimmed labels

I recently introduced some changes in Settings that happen to trigger this 100% (Date & Time panel with automatic timezones on).

I'm filing this is issue separatedly because I think it's different than #3833 (closed). This report is about rendering artifacts beneath labels, not blurriness.

Have a look:

Interestingly, the inspector's magnifier doesn't seem to render the GtkLabels themselves with these artifacts:

As I write this bug report, I realized that I can reproduce this with any Text node in the inspector by resizing it. Take this text node for example:

text {
  color: rgba(0,0,0,0.8);
  font: "Cantarell Bold 11";
  glyphs: "Data & hora";
  offset: 0 14.418;
}

If I resize the inspector window, I can see rendering artifacts there: