Now we are getting close to a a pausible ATC scenario.
Santos Dumont : By "lost comm " you mean LAST frequency, it makes more sense . The vast majority of comm failures toiday ( if not all ) are ears or finger trouble on a frequency change .
What is missing is the coordination level on the 738 with Brasilia. If the estimate (or the auto ACT if they have one ) was a stable level but the a/c was still climbing instead then you have it.
As I said earlier , if indeed it is true that the Legacy Xponder went on SBY on its own AGAIN and on a new set, then Honeywell and many E145 operators are going to be in sh.... very soon.
ICAO versus FAA rules , the problem is old and has no solution. There are so many FAA procedures completely at odds with ICAO that it was only a matter of time before they kill someone.
Another chain of unprobable events and Murphy's law active again.