It's a transparency issue when placing a native Illustrator file with a transparent background into an Indesign file. A white background is added to the placed file . This happens with ANY InDesign file (New or existing) and ANY Illustrator file (New or Existing).
When you place the Illustrator file, choose Show Import Options (Shift + Open is the shortcut). Confirm that Transparent Background is checked.
I knew I should have come here first
Thanks for the quick and absolutely correct answer.
Any idea how this would have changed? I did not change it.
I looked for a way to change it accidentally. Spacebar will check/uncheck a checkbox if you are on the field. But I had to press Tab four times to move to the Transparent Background field before hitting Space would uncheck it.
So no, I don't have any viable ideas for how it might have changed.
But I'm glad you are up and running!