This is another problem that goes away if I --disable-font-backend at runtime.

In `emacs -Q', most font-lock faces are for some reason in a different
font to the default font.

This is hard to describe, so I attach a screenshot (I hope it makes it
through the list OK).

If I do
M-x customize-face font-lock-string-face RET

then the "sample" in the foreground section looks right, but the
"sample" next to "Hide Face" is using the wrong font, as you can
hopefully see.

Probably more information is required to diagnose this, but I don't
know what. Let me know. TIA.