3.24.36 reftest previously worked but now fails: label-background.ui rendering difference

Steps to reproduce

Build and run build-time tests in Debian unstable (rolling release) on x86_64

Current behavior

Diff:

Output:

Reference:

There is a rendering difference in the blue channel in the first column of pixels adjacent to the left vertical part of the H.

Expected outcome

Reftest renders identically to reference

Version information

3.24.36 (specifically Debian's 3.24.36-1 package)

Additional information

The same GTK source code passed this test when it was first built, but now fails. Presumably the test failure was triggered by some external package. We recently upgraded to fontconfig 2.14.1, so it could be that?