Missing ranked rewards

When I logged into Marathon to check the ranked rewards, I was really excited because I spent a lot of time reaching Pinnacle 1. However, when I checked my codex, I only received the rewards up to Diamond 3. I reached Pinnacle 1 earlier this week, and afterward I was helping my friends rank up, which caused me to drop down to Platinum 1. My understanding was that ranked rewards would be based on the highest rank achieved during the season, not the rank you ended the season at. Could you please help me? Reaching Pinnacle 1 was my final goal for Season 1. and right now I feel so defeated cause i spent all that time and don't have the rewards to show for it. (I have one screenshot i took early in the week when i reached it but that's it)