So my question is why do I see this with 777's . Enough times to notice it and not really notived with other types.
If you mean why don't you see an aircraft symbol for many 777s, it's because RadarBox doesn't make use of heading data that it decodes, only track data.
If you mean why do those particular aircraft send heading and not track, I have no idea :-)
I'm wondering if there is a subtle bug somewhere related to this.
I've seen a couple of cases, I think with either UAL or AAL aircraft, where initially I see a track with the aircraft symbol pointing where it should.Then, because of the topography around my house, the track times out and the symbol changes to the circle with x inside it. Before the final timeout occurs to remove the symbol from the map, reception returns and the track reappears. The aircraft is still transmitting the same ADS-B frames, but from that point on the circle with x symbol remains until I lose coverage completely.
It's certainly very infrequent, I've seen it twice now that I can be sure about.