Hi,
I've not been able to fix it.
However I did some troubleshooting, including doing a simple blink program using sleep instead of delay, and within this program make a call to a method of another class which also runs a low power sleep, and that doesn't seem to be a problem.
That definitely looks to be a combination of the two timed wakeups with Sigfox, not sure where exactly.
Now I have to debug Sigfox class and put some markers to exactly identify where it blocks.