A recent update breaks my fonts for most programs, making all characters show as empty boxes. This happens inside and outside tmux, across various terminal emulators, etc. It does not occur in deluge-gtk or my TTY. I experimented with rolling back and doing a partial upgrade from there. With this list of packages to upgrade, the issue comes back.

The following packages will be upgraded:
   font-terminus        4.40 → 4.47     /gnu/store/a3sksxcg43z7scag56mgg32mjnw2k6fd-font-terminus-4.47                                                       
   font-gnu-unifont     11.0.03 → 12.0.01       /gnu/store/ypljs2fyv33f6l55mj6spha7w2pxlr3w-font-gnu-unifont-12.0.01                                         
   mpd  0.21.4 → 0.21.5 /gnu/store/dgj45pd8cg9n9fs95mlw8yprkdrsb4fz-mpd-0.21.5
   gnunet-gtk   0.10.1 → 0.10.1 /gnu/store/3vz8z6wn0cdrfsbz9yxclzyn7vnb5w81-gnunet-gtk-0.10.1                                                                
   gnunet       0.10.1 → 0.10.1 /gnu/store/a8jahsmhk21vz350izp2zfn1dbb3bc3j-gnunet-0.10.1                                                                    
   gimp 2.10.8 → 2.10.8 /gnu/store/81z6bjvd9rx05l4hviisc98cirrn57z9-gimp-2.10.8
   ffmpeg       4.1.1 → 4.1.1   /gnu/store/4hcr7ygdhaxws6q5dj806cbvq0dkfgkw-ffmpeg-4.1.1                                                                     
   youtube-dl   2019.02.18 → 2019.03.01 /gnu/store/1glmmv0lv1szipgk3ikbjdgp9xnncvrm-youtube-dl-2019.03.01                                                    
   qucs-s       0.0.20 → 0.0.20 /gnu/store/vf4pbqbaxq87rgjxw98alhgcd4dmahyz-qucs-s-0.0.20                                                                    
   mpv  0.29.1 → 0.29.1 /gnu/store/xyfjhq1mcq4d4ijy65fxishz467r37kl-mpv-0.29.1
   vinagre      3.22.0 → 3.22.0 /gnu/store/gp3xi8zgxdxjiidk5b4wkq34vn08jv58-vinagre-3.22.0                                                                   
   zathura      0.4.3 → 0.4.3   /gnu/store/92pkqiqsa4ngm70mw8a4qn9jr7azj2ap-zathura-0.4.3                                                                    
   xinit        1.4.0 → 1.4.1   /gnu/store/d01b76z9kdyyajyxqln2m56i8ghdkdys-xinit-1.4.1                                                                      
   xdg-utils    1.1.3 → 1.1.3   /gnu/store/gmzvdpqzcrvf06pjb3594gvhc46zyd12-xdg-utils-1.1.3                                                                  
   ruby-pry     0.11.3 → 0.11.3 /gnu/store/vpv29xhpv7j1mi0plis1svqr7sf0n48j-ruby-pry-0.11.3                                                                  
   readline     7.0.5 → 7.0.5   /gnu/store/si88gb25621k0w60kij5lxksn8l24664-readline-7.0.5                                                                   
   python-glances       3.0.2 → 3.0.2   /gnu/store/9pa0x2s8jnldjkij7gb4yrinm0ccyj5q-python-glances-3.0.2                                                     
   pulseaudio   12.2 → 12.2     /gnu/store/hd3x89yknm4k6k4gw16jy5xk7idq1sxf-pulseaudio-12.2                                                                  
   pcmanfm      1.3.0 → 1.3.0   /gnu/store/b0585azqb0wibpx7qn6gr740f8fh6cw5-pcmanfm-1.3.0                                                                    
   minetest     0.4.17.1 → 5.0.0        /gnu/store/7n1dvbxqd2kk75sj4zi4snzw7d5y63cv-minetest-5.0.0                                                           
   fontconfig   2.13.1 → 2.13.1 /gnu/store/66jfnfgca7yi6xmpw6ax86cldvr016ia-fontconfig-2.13.1                                                                
   font-adobe-source-han-sans:jp        1.004 → 1.004   /gnu/store/zaj1ias1gdz3jmsgvhvn0c706aqw32b1-font-adobe-source-han-sans-1.004-jp                      
   dconf        0.28.0 → 0.28.0 /gnu/store/x6dgfq8b382jgciq24zcxlgpgxfxaxk2-dconf-0.28.0                                                                     

It is hard to say what is to blame here. Three separate font packages are being upgraded as well as fontconfig. Attached is a screenshot showing what this breakage looks like. I worry that holding back fontconfig might  lead to issues with the other fonts upgrading. I don't know if those fonts all updated due to universal font changes.

$guix --version

guix (GNU Guix) 2b613a1a5d4d41b0b5d1f6ea7254585be0c209fa