The internal reward structure of Legendary Tasks was updated to a more robust system in 4.7.
This didn’t affect the possible Rewards or the drop rates but it did mean that if we had 2 players, one on 4.6.5 and one on 4.7 who collected the same Task they received different rewards because they were on 2 different systems which grabbed the rewards differently. So instead of the LT Rewards being pulled once for the entire Guild, it was pulled twice (once for the 4.6.5 players and once for the 4.7 players).
This was fixed before the update released on PC/mobile, however, unfortunately, the update was released on console before we could push the fix live so this should have only affected console players - unfortunately, that was already a moving train we couldn’t stop 
The issue was fixed in roughly 3 hours of the update hitting console by us preventing players on 4.6.5 contributing to or receiving Guild Tasks until after they updated to 4.7. No rewards were missed - they were all sent to their mail box once they’d updated to 4.7.
The only issue was situations like the OP’s where unfortunately, one side got a mythic while the others only got a Legendary