You can see this in text-scaled buffers when attempting to pad a string to a specific pixel boundary.  If a pixelwise specified-width space is inserted after a string with variable-pitch glyphs, it doesn't honor the net pixel width.  That same string padded using min-width with the same number of pixels works fine.

I've attached a -Q reproducer which I've run on 30.1 and a recent master with the same results on macOS.  On master, I run as "src/emacs -Q -l repro-variable-pitch-pixels.el".

The reproducer shows that a variable-pitch string padded to a specific pixel width using string+specified space is unreliable vs. string with a min-width display property.  Fixed-pitch glyphs seem fine and everything works at scale 0.  There are three buffers created: scale 0, scale -1, scale +1.

I tried to find where in xdisp.c or in ns*.m this would manifest, but it wasn't immediately obvious.  I don't have access to another platform this week, so I do not know if this is NS specific or also manifests with other toolkits.

-Stephane