Just to clarify, you mention this happened back during the Season 11 split. That would have been back in December of last year, is that right?
Or were you referring to the more recent Season 12 split from last month?
Do keep in mind you wouldn't receive those ranked rewards until the season ends, not just the split. You would then be able to use the trail for the next season.
If this happened with the split last month, you won't be seeing the rewards yet as the current season has not ended, however you should see them show up in-game with the upcoming season.
If this was during the Season 11 split back in December that will limit what we're able to investigate due to how much time has passed, but please create a bug report here so we can gather as much information as possible to investigate, thanks!