Community Manager@B3rkyBerk This has been an issue for a while. Sadly, there won't be a one-step fix for it at the moment as there hasn't been an update in a while. The most common ways to resolve it are as follows: I added some links with steps that should help with it as well.
Reload Save: Try reloading a save file from before you created the 6th strike team, then create the teams again.
Check Online Status: Make sure you're online when creating the teams, as some achievements require an internet connection to register.
Retire and Recreate Teams: Some players have had success by retiring their existing teams and then recreating them.
One step I always do is load the save (if you can) before you get it, or a few minutes before, save it as a new save file, then go back through the mission you were on to see if that pops the achievement.
