Number in front of callsign

[Henning]

I am experiencing a behaviour that I never saw before for the second time. There is a flight on my flight list with a callsign of TGW651. But on the radar screen there is a number in front of the callsign and this number is changing every few seconds. It's switching between 0 and 5. So for a couple of seconds the displayed callsign is 0TGW651 and then for a couple of seconds it's 5TGW651.
I also experienced that yesterday for the same aircraft but for the callsign TGW628.

The aircraft symbol is also facing north all the time.

The aircraft has the following details:
Mode-S: 7C6C27
Reg: VH-VND

Has anyone else experienced this for the same or a different aircraft?

[Henning]

Here are some screenshots that show what happens (it happened again today).

The first screenshots shows flight TGW618 with as 0TGW618, the second screenshot shows it as 5TGW618 and the third screenshots shows the same flight in the list listed as TGW618 (without any numbers in front).

[Runway 31]

I would suggest that the data coming from the aircraft regarding the flight number is constantly changing due to some malfunction of its equipment.  I havent seen the flight number changing before but have seen the hex code constantly changing.

Alan

[Henning]

But why is the callsign displayed correctly in the flight list? I would expect to see it change there as well then.