Mine just dropped. It may have been the history museum but nothing special. I got a different exotic brand as well. One should drop just keep at it. I did not buy a season pass & only have WONY.
You do not need a season pass to get free Waveform. Only level 90 this season. After that you can get it in any loot.
Or, maybe, it's already in basic loot, since season is over.