When I select a legacy deck to overwrite the current event deck, it complains about invalid cards and won’t replace it.
I can select “use this deck” and it will use it, but the original choice for the event deck remains.
I’m having a similar issue, though unrelated to legacy. I can’t use the copy deck option once I have an event deck set in Training Grounds. Also, after my four battles I tried making an event deck out of cards I don’t have and now I can’t even edit that event deck, it just flashes the overwrite deck dialog whenever I click the deck. Weird.
The copy deck feature is what I’m talking about. Didn’t try to add cards I don’t have. That’s really weird.
I wonder if the bug or problematic functionality is tied to the recent change in what sets make up Standard.