Hi, what's empty is the numbers you have entered in the "Group members" section, not the log (the log obvius is empty at this time, I had no time to try this

).
I mean: You put a number to filter in "Group members" (i.e. 1004), you save it, all OK, it's registered. Then you exit avast dashboard, come again into it, and you go to SMS and call filters, press the log button for this group (the log is empty, of course), you go back and the number you have entered previously (1004) disappears of the "Group members": you now have a group members without any number to filter.
Thanks.