busctl along with dbus-monitor do show the signals, it seem to be just Qt that does something differently:
[nemo@Sailfish ~]$ busctl --user --verbose monitor --match "type='signal',sender='uk.co.piggz.amazfish2',interface='uk.co.piggz.amazfish2'"
Monitoring bus message stream.
‣ Type=signal Endian=l Flags=1 Version=1 Priority=0 Cookie=2
Sender=org.freedesktop.DBus Destination=:1.545 Path=/org/freedesktop/DBus Interface=org.freedesktop.DBus Member=NameAcquired
MESSAGE "s" {
STRING ":1.545";
};
‣ Type=signal Endian=l Flags=1 Version=1 Priority=0 Cookie=3
Sender=:1.546 Path=/application Interface=uk.co.piggz.amazfish2 Member=informationChanged
UniqueName=:1.546
MESSAGE "is" {
INT32 0;
STRING "xxx";
};