Strange because this time I added the missing package. My only change in the deb building process:
Package: siril
Architecture: any
Depends: ${misc:Depends},
${shlibs:Depends},
siril-common (= ${source:Version}),
librsvg2-2,
gvfs-backends
And in the flatpack we already have:
/* Needed for gvfs to work */
"--talk-name=org.gtk.vfs", "--talk-name=org.gtk.vfs.*"