There's a closed pull request on the void-packages repository

Adds xdg-desktop-portal as a dependency for flatpak
https://github.com/void-linux/void-packages/pull/39844

Description:
Flatpak relies on xdg-desktop-portal heavily, why it isn't a dependency is unknown to me.

Additionally, it would make sense to add xdg-desktop-portal-gtk as a dep, since that's needed for GTK theming and a native filepicker in the event there isn't another xdg-desktop-portal implementation that has one.