On Fedora, $SNAP points to the snap’s directory under /var/lib/snapd/snap/$SNAP_NAME/$SNAP_REVISION. If you list the contents of the mount point for a connected content interface, it’s empty. However, listing the contents of the mount point under /snap/$SNAP_NAME/$SNAP_REVISION/ does show the contents.
I’ve been able to work around this in the snapcraft-desktop-helpers by resetting $SNAP to /snap/$SNAP_NAME/$SNAP_REVISION. However, I think there’s an underlying bug here and should probably be fixed in snapd.