Not enough evidence to be sure there's a bug, but in case others see it:
Two WAC124 and one DIR882 (all MT7621 based) upgraded. All had issues keeping the LAN1-LAN4 interfaces up after boot - they would go down and up every 10-60 seconds. Behavior would clear up after about 5 minutes.
Same w/ 5GHz on one of the wac124's (others not tested), though this was less frequent and more random in timing. (not a DFS channel)
All had 'sysupgrade -n', so a clean configuration. LAN1-4 behavior happened with the initial configuration after upgrade. Three different USB-ethernet adapters used to connect, so do not think that is the issue.
Could this be related to the throughput issues others are seeing?
Reverted to .3 for the main unit.
These devices have been used on many release (mainline, snapshot) without issue. But it could be other issues, such as laptop kernel changes.
Many thanks for all the work on a very appreciated project.