Bug : One And Only Trophy

I encountered a very weird bug when I was going for the one and only trophy.

I completed the arena on Hardcore difficulty and the trophy popped but somehow it didn't register as part of the earned trophies.(Inside the trophy list for ME3, it is still showing up as 'Not Earned'.)

I replayed the arena multiple times on Normal & Hardcore difficulty but now the trophy is not popping up.

Please help as I was going for completion across all the 3 games.