I finished season 3 as a plat tank and Gold dps but i got Gold rewards.This happened to me in season 2 ranked rewards too
The Devs have acknowledged this bug: Overwatch 2 Known Issues - April 11, 2023. However, we still need to keep spreading the word about this bug so that they actually fix it unlike the bugged career profiles and missing end-of-season rewards from other previous seasons like Season 36. Here is a link to my post in case you’d like to read more about this issue: Comprehensive Thread Of All Top 500/Competitive Bugs. I’ve compiled a list of posts regarding similar bugs under: Incorrect End Of Season Competitive Rank And Rewards (Seasons 1-3). Please help in spreading the word about this megathread and other related threads, we must demand the Developers to give us the rewards we rightfully earned.