Was prompted to create an account to save deck.
You must be logged in if you wish to post to the forum.
However after I created this account, the deck list was cleared. Is this intended? Is there any way I can reclaim this deck list?
Unfortunately no. If you leave the deck builder page without submitting it then you can't reclaim it.
To be fair we do have a message on top of the deck builder if you are logged out:
"You must be logged in if you wish to submit a deck you create.".