Qualcomm's Thread implementation sends a short address in the beacon instead of the extended address. Section "3.2 MAC Layer" of the Thread 1.1.0 Specification modifies section "7.5.2.4 Beacon generation" of the 802.15.4-2006 Specification to ALWAYS send an extended address. See Table 3-2. Modifications and Statements to Chapter 7 and Annexes of IEEE802.154 ( https://standards.ieee.org/findstds/standard/802.15.4-2006.html )
From the Thread 1.1.0 Specification:
"The address used in the Source Address field of the beacon frame generated by a Thread device SHALL contain the value of aExtendedAddress for all values of macShortAddress."
The ramifications are devices that conform to the Thread Specification reject the non-conforming beacon and are unable to join networks where a QCA4020 is acting as commissioner.
FYI, I first notified Qualcomm of problems with the beacon back on June 3, 1019.
https://developer.qualcomm.com/forum/qdn-forums/hardware/qca4020-qca4024/66927
I meant to add that git commit 9a0b838e5bcbf70049a30aaf68abd22d231f5785 from the OpenThread repository, the version used to build the OpenThread Library used by QAPI SDK, does NOT exhibit this problem. When I check out commit 9a0b838e5bcbf70049a30aaf68abd22d231f5785 and build it for the NXP USBKW41Z the beacon is correct and conforming devices can successfully join the network.
We have created a SF# case and are working internally to fix the issue in next release.