@james nothing font related in browser console, Nextcloud or Collabora logs. However what I figured was that the custom fonts at /app/data/fonts are never loaded. Seems like the directory is not even beeing looked at.
root@e5fa3d92-8f21-4d3a-bfed-4f24403d76f3:/app/code# fc-cache -v
Font directories:
/root/.local/share/fonts
/usr/local/share/fonts
/usr/share/fonts
/root/.fonts
/usr/share/fonts/X11
/usr/share/fonts/cMap
/usr/share/fonts/cmap
/usr/share/fonts/opentype
/usr/share/fonts/truetype
/usr/share/fonts/type1
/usr/share/fonts/X11/Type1
/usr/share/fonts/X11/encodings
/usr/share/fonts/X11/util
/usr/share/fonts/cmap/adobe-cns1
/usr/share/fonts/cmap/adobe-gb1
/usr/share/fonts/cmap/adobe-japan1
/usr/share/fonts/cmap/adobe-japan2
/usr/share/fonts/cmap/adobe-korea1
/usr/share/fonts/opentype/urw-base35
/usr/share/fonts/truetype/dejavu
/usr/share/fonts/truetype/open-sans
/usr/share/fonts/type1/urw-base35
/usr/share/fonts/X11/encodings/large
/root/.local/share/fonts: skipping, no such directory
/usr/local/share/fonts: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts: skipping, existing cache is valid: 0 fonts, 6 dirs
/usr/share/fonts/X11: skipping, existing cache is valid: 0 fonts, 3 dirs
/usr/share/fonts/X11/Type1: skipping, existing cache is valid: 35 fonts, 0 dirs
/usr/share/fonts/X11/encodings: skipping, existing cache is valid: 0 fonts, 1 dirs
/usr/share/fonts/X11/encodings/large: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/X11/util: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cMap: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cmap: skipping, existing cache is valid: 0 fonts, 5 dirs
/usr/share/fonts/cmap/adobe-cns1: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cmap/adobe-gb1: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cmap/adobe-japan1: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cmap/adobe-japan2: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/cmap/adobe-korea1: skipping, existing cache is valid: 0 fonts, 0 dirs
/usr/share/fonts/opentype: skipping, existing cache is valid: 0 fonts, 1 dirs
/usr/share/fonts/opentype/urw-base35: skipping, existing cache is valid: 35 fonts, 0 dirs
/usr/share/fonts/truetype: skipping, existing cache is valid: 0 fonts, 2 dirs
/usr/share/fonts/truetype/dejavu: skipping, existing cache is valid: 8 fonts, 0 dirs
/usr/share/fonts/truetype/open-sans: skipping, existing cache is valid: 13 fonts, 0 dirs
/usr/share/fonts/type1: skipping, existing cache is valid: 0 fonts, 1 dirs
/usr/share/fonts/type1/urw-base35: skipping, existing cache is valid: 35 fonts, 0 dirs
/root/.fonts: skipping, no such directory
/usr/share/fonts/X11: skipping, looped directory detected
/usr/share/fonts/cMap: skipping, looped directory detected
/usr/share/fonts/cmap: skipping, looped directory detected
/usr/share/fonts/opentype: skipping, looped directory detected
/usr/share/fonts/truetype: skipping, looped directory detected
/usr/share/fonts/type1: skipping, looped directory detected
/usr/share/fonts/X11/Type1: skipping, looped directory detected
/usr/share/fonts/X11/encodings: skipping, looped directory detected
/usr/share/fonts/X11/util: skipping, looped directory detected
/usr/share/fonts/cmap/adobe-cns1: skipping, looped directory detected
/usr/share/fonts/cmap/adobe-gb1: skipping, looped directory detected
/usr/share/fonts/cmap/adobe-japan1: skipping, looped directory detected
/usr/share/fonts/cmap/adobe-japan2: skipping, looped directory detected
/usr/share/fonts/cmap/adobe-korea1: skipping, looped directory detected
/usr/share/fonts/opentype/urw-base35: skipping, looped directory detected
/usr/share/fonts/truetype/dejavu: skipping, looped directory detected
/usr/share/fonts/truetype/open-sans: skipping, looped directory detected
/usr/share/fonts/type1/urw-base35: skipping, looped directory detected
/usr/share/fonts/X11/encodings/large: skipping, looped directory detected
/var/cache/fontconfig: not cleaning unwritable cache directory
/root/.cache/fontconfig: cleaning cache directory
/root/.cache/fontconfig: invalid cache file: 9b5b740b6269753c3eba86fc14d9c368-le64.cache-9
/root/.fontconfig: not cleaning non-existent cache directory
fc-cache: succeeded
I have to hide the original app so users only use my custom link.


Update showed up and worked as expected. Thank you 
