I found this community forum post: https://us.forums.blizzard.com/en/hearthstone/t/im-having-trouble-claiming-the-deck/27567/9
User Daniel and user Norteklaser268 both said the different regions are treated as different accounts.

Hi Stonekeep
I know this post I’m replying to was some days ago, but I have conflicting info on this. In the blue test it really says ‘once per account’. But people reading this post please see this link: https://us.battle.net/support/en/article/260329 . There it says ‘once per play region’, which would be 3 times per account, right? Just wanted to direct your attention to this ambivalent info.