Comment 36 for bug 264254

One possible reason could be the ordering of the fontconfig rules.

/etc/fonts/conf.d/README reads:

 00 through 09 Font directories
 10 through 19 system rendering defaults (AA, etc)
 20 through 29 font rendering options
 30 through 39 family substitution
 40 through 49 generic identification, map family->generic
 50 through 59 alternate config file loading
 60 through 69 generic aliases, map generic->family
 70 through 79 select font (adjust which fonts are available)
 80 through 89 match target="scan" (modify scanned patterns)
 90 through 99 font synthesis

I used 11-lcd-filter-lcddefault.conf to match "system rendering defaults (AA, etc)"
The user ~/.fonts.conf file is loaded in 50-user.conf so one or more rules between 11 and 50 may interfere.

10-antialias.conf
10-hinting.conf
10-hinting-medium.conf
10-no-sub-pixel.conf
11-lcd-filter-lcddefault.conf
20-fix-globaladvance.conf
20-unhint-small-vera.conf
25-ttf-arphic-ukai-render.conf
25-ttf-arphic-uming-bitmaps.conf
25-ttf-arphic-uming-render.conf
30-cjk-aliases.conf
30-debconf-no-bitmaps.conf
30-defoma.conf
30-metric-aliases.conf
30-urw-aliases.conf
35-ttf-arphic-ukai-aliases.conf
35-ttf-arphic-uming-aliases.conf
40-nonlatin.conf
41-ttf-arphic-ukai.conf
41-ttf-arphic-uming.conf
45-latin.conf
49-sansserif.conf
50-user.conf
51-local.conf
52-languageselector.conf
53-monospace-lcd-filter.conf
60-latin.conf
64-ttf-arphic-uming.conf
65-fonts-persian.conf
65-nonlatin.conf
65-ttf-thai-tlwg.conf
69-unifont.conf
70-yes-bitmaps.conf
75-ttf-arphic-ukai-select.conf
80-delicious.conf
90-synthetic.conf
90-ttf-arphic-ukai-embolden.conf
90-ttf-arphic-uming-embolden.conf
90-ttf-thai-tlwg-synthetic.conf