EDIT: interesting, relog helped... almost. Now i can claim them on all toons BUT on the one i've unlocked them
Short answer: To get Celestial (R), you need to have a Mythic (R) in your possession.
You go to claim agent to get the mythic one and then use the mythic one to get the celestial one. You do not have a mythic one when you try to claim the celestial one for the 1st character (because you upgraded the original mythic one to celestial to get celestial unlock). Your other character has that mythic one in their possession. Hence, you need to claim that Mythic (R) back to exchange for the Celestial (R).
*** The game can read your mind. If you want it, you won't get it. If you don't expect to get it, you will. ***
"If if that was the case, if it was something then I probably was chasing. I would have never gotten it. That was the whole point, if you chase something, then sometimes you never get it. uh huh if you put forth to work and all the attitude, next you know it's bestowed upon you." -- Michael Jordan
... because you upgraded the original mythic one to celestial to get celestial unlock). Your other character has that mythic one in their possession. Hence, you need to claim that Mythic (R) back to exchange for the Celestial (R).
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You will need that mystic to claim the celestial
You go to claim agent to get the mythic one and then use the mythic one to get the celestial one.
You do not have a mythic one when you try to claim the celestial one for the 1st character (because you upgraded the original mythic one to celestial to get celestial unlock). Your other character has that mythic one in their possession. Hence, you need to claim that Mythic (R) back to exchange for the Celestial (R).