Prized Amalgam stops respawning from the graveyard after a long game.
It’s happened multiple times during Hallow’s Eve. I wondered if it was because of some manner of exile effect I wasn’t noticing, but I’m sure that this is not the issue now.
I see what it is. If you decline to put the Amalgams back into play one time, you never get asked if you want to do it again. This is not the way it used to work, and it’s not intuitive.