From what I have been able to find, the way to progress this is by doing the optional objective in the middle of a lawless frontier activity. I've done numerous activities, and have claimed the chest from this optional activity multiple times (even just did a solo run on expert level and was the only person - perhaps obviously - to do anything in the optional objective). But I'm stuck at 1/3. This doesn't make any sense, unless I'm not understanding what constitutes completing an optional objective in a Lawless Frontier activity. As an aside, is there any chance at all of getting a search function in these forums? It's really annoying to have an issue and not be able to search first to see if there's already a fix or something that I've just missed. (That's been a long-standing issue for me, but I don't think I ever actually asked about it)