I purchased a dye and it's acting as if it needs to be purchased per champion, which doesn't seem like that's how dyes are expected to work since dyes from the season pass seem to be unlocked for all champions. Side note, this is the first dye I've purchased outside of the season pass.
I purchased the one dye three times: first time it worked as expected; second time because I thought maybe I didn't actually purchase it since it had the shop price tag on it when I tried to apply it to another champion and the third time to record it.
I originally had Gwen, Barnard, and Rees in solid red and purchased the red/grey. I first applied the dye to Gwen, second time I applied it to Barnard, third time was the test on Rees. You can see on the third time the purchase appears to remove the shop price tag and I can apply it to Rees, but when I try to apply it to Brel it doesn't let me apply it to him and the shop price is back and the purchase spindle pops up ready for me to buy it again.
I even logged out and logged back into see if the shop price tag had dropped off, but it was still there. I really hope that's a bug and not the intended design.