Fontconfig in gnome platform has the same rendering for different hintstyles
Font rendering in gnome-based flatpak applications looks the same with hintstyle set to hintfull or hintslight. There is a difference in these styles in regular fontconfig. For tests i changed fontconfig settings using ~/var/app/config/fontconfig/fonts.conf file. There is a difference between hintnone and hintslight/hintfull - so, i assume flatpak applications uses settings provided by ~/var/app/config/fontconfig/fonts.conf (although, the issue can be with gnome overriding hintslight with hintfull but not overriding hintnone for some reason). I tested fractal on 3.28 platform, fractal-nightly on master platform and podcasts on 3.28 platform - results was the same.
Flatpak with qt5 app Focuswriter correctly renders font with hintstyle set to hintslight.
Fractal flatpak with hintslight/hintfull
Polari with regular system fontconfig 2.13.0 on fedora 28, hintfull
Polari with regular system fontconfig 2.13.0 on fedora 28, hintslight
Focuswriter (qt5 app) flatpak with correct hintslight rendering