Thanks a lot Andy, and special thanks too for the link to this thread : z-wave-issues-with-firmware-update-30-3.
It looks like they have clearly isolated the incompatibility issue, related to the (possibly illegal) use of multiple channels with endpoints <>0 by FIBARO devices, including FGK-10x.
I also posted in this thread to remind SmartThings development that the issue is NOT limited to endpoint 1, but also concerns endpoints 2, specifically used by FIBARO for Temperature reports.
The trace you posted is definitely consistent with that since no multi-channels reports are present in it.
If the analysis in that other thread “Z-Wave Issues with Firmware Update 30.3” is correct, then there is nothing I can do anyway within my custom Handler to work around this issue, since the temperature reports are all filtered out by the Z-Wave layer of the new v2/v3 Hubs firmware, BEFORE my handler gets control.
We will have to wait and pray, hoping that SmartThings will backtrack, rather than making furious all users of FIBARO (and others) devices 
.