I have finished all Tier 1 objectives for the East Coast Mainline, yet the reward and tier 2 are still locked.
I don't know if it's working as it should and I just misunderstood the mastery HUD, but I think I'm having issues too, in my case with the festive 2023 mastery thingy that just came out for TSW3. The rewards on the right side of the screen have an explosive and a radioactive material warning signs drawn but despite their tier being unlocked they are nowhere to be found in the Livery Designer? Pretty sure some other stickers that were not there before have appeared, 'cause I have like a new "deer warning" looking one?
Mastery is the biggest joke in the game so far. It almost rarely works, and in the rare occassions when it does, there's no actual guarentee that the rewards unlock either.