The achievement for winning the constructor's championship doesn't unlock when you complete it. I have won it twice in My Team, and neither time did it unlock.
Ideas
Okay. Have you tried winning in t in regular career mode? Of not it might be worth trying it there just incase
Cpayne32 I haven't tried it yet, however I asked some other people I know who DID try it, but they also did not receive the trophy / achievement even though they won the constructors championship. Some of them did get the trophy / achievement (in MyTeam) so I guess it's just luck right now. Let's just wait until EA comes with more info about this or has fixed it. EA_Groguet said that he doesn't have any news about it yet but they are investigating it :)
Yeah. Just giving a suggestion to try in the interim but there are a few other bugged achievements as well