Same issue happened to me. Got the 2k achievement reward, but my inventory was unknowingly full when I accepted the reward. I received the Zenith skin (which went to overflow inventory) and the passive account rewards, but the large achievement chest was lost and never showed up in my inventory nor the overflow inventory.
This is definitely a bug, I’m 100% sure I did not sell it nor delete as the first thing I did when I accepted the reward was to try to find the large achievement chest.