One odd thing is that FW updates do not seem to cause this issue even though the SMs will upgrade first and then the AP will upgrade and reboot. Maybe the SMs did not connect to the AP before the AP rebooted and the trigger is that the SMs need to be registered first?
Here are some posts I made about similar issue, but with different triggers; some of which have been since fixed. So I wonder if the underlying issue is still the same, but Cambium has fixed the triggers in some of the below cases.
Erratic uplink performance under MU-MIMO with 5.10.1-RC14 - Products / ePMP Beta - Cambium Community (Changed AP from Beamforming to MU-MIMO)
Reduced uplink rates on 5.10.0-rc35 - Products / ePMP Beta - Cambium Community (Noticed after turning on/off Asymmetrical UL. This trigger seems to have been fixed since then)
Scaling issue once I reach 12 SMs on an AP? - Products / ePMP - Cambium Community (Same overall result, but was fixed in later FW updates)