[Baertram]

I had the same problem which couldn't be solved in the past.
So I wrote a workaround at player activated event callback function, which get's the actual group size and checks if I'm in the group member table:

-- Get group Size
    local groupSize = GetGroupSize()
    if groupSize <= 1 then
        d(">> You are not in a group")
        return
    end
 
    local charName
    local playerName = GetUnitName("player")
    -- Cycle through group and check their "unitTags"
    for i=1, groupSize, 1 do
        charName = GetUnitName(GetGroupUnitTagByIndex(i))
        if charName == playerName then
            d("You are in a group!")
                return -- break here
        end
    end

Edit:
Oh and for all other situations the callback function for event EVENT_GROUP_MEMBER_JOINED used the same check, and it fired correctly for me (couldn't test it in the past days though!)