Oh you’re talking about FA anonymizing MLAT results …
FlightAware MLAT Network Announcement
This is normal and has nothing to do with what airspy produces as messages.
You’ll have aircraft with similar or the same altitude showing as ModeS, that will be the “real” aircraft. (unless you participate in another mlat network with full results, then the real aircraft will be MLAT as well)
Might be the compiler version, i’ll look into it.
Had to change how i build as one of my RPi bit the dust.
@JRG1956
Actually, i recompiled RC19 with compiler i was using before.
See if that changed anything CPU wise, thank you.
Apart from that i’d highly recommend -C, it works very nicely now 
And if you have issues with altitudes, just set -P 8 or something to keep that in check.