Not entirely sure if this is a bug or an intended design, but going to add it anyway.
So I’ve personally unlocked Pit Tiers 1 through 7. However I joined a friend who had already advanced to Pit Tier 50, so I joined them in a Pit Tier 50 and we managed to complete it successfully, we continued up to about tier 56 or so. Then just last night he invited me to a level 60 Tier, which we also managed to complete.
So now when I select which Tier I want to enter for the Pits I have 1 through 7, 51 through 58, and now 61. I don’t have access to anything between 8 and 50, and I don’t have access to tiers 59 and 60.
Now this isn’t a big deal mind you, just thought it was kind of strange. You would think completing a higher tier would unlock all the ones before it.