I had a pop out in game that I got ETC Band Manager card for free, but it never went into my collection. The pop out came after the Thunderbringer card one and the second card was normally added to my collection.
Free legendaries are only granted automatically for a limited time, since ETC comes from the latest standard expansion maybe it’s no longer automatic despite the pop up, can’t tell which one would be the bugged one.
If that’s the case, the manual way to receive such legendaries for “free” is to get a pack from their expansion (Festival of Legends) for ETC and go to your pack opening screen. You should receive the card before opening the pack.