I recall a bug report about this from several years ago. Similar issue, part of the guild getting a mythic troop, the others getting a legendary troop, from the same legendary task. I think it depended on when the troop got collected from the mail system, not sure about explanation given. Might require quite a bit of digging in old forum threads.
This might actually be related to other weird server issues that have been popping up today, common factor being timing: