I think the answear to that question is in this topic:
http://community.cambiumnetworks.com/t5/ePMP-2000-and-1000/ePMP-Release-3-5-is-now-available/td-p/77234/page/3
Starting on page 3.
The reason is:
"There were several fixes related to RSSI calculation accuracy introduced in 3.x firmware.
Could you please check whether RSSI wasn't changed?"
-Fedor
So basically you didn't realy lose any signal, just calculation accuracy is now better.
Regards