Was the white box inadvertantly added to the file in Illustrator, and then added to the library? Look at the artwork thumbnail in the CC Library. Do you see checkerboard directly behind the object, or white?
In my example below, the group of trees has a white background, but the four individual trees do not.
The problem occurred with version 20. I have tried several times. I have created several objects, I also tried with vector objects previously they did not have this problem.
Version 20? Do you mean InDesign 2020? That is version 15. Be sure you are all patched up. The current 2020 version is 15.1.3.
I just tried this in 2020 and it worked as expected. Did you try in a brand new file?
I'm quite sure that the following happened:
The last time you placed an AI file or a PDF file you disabled option Transparent Background in the Import Options.
Because the assets marked as AI in your CC Libraries are nothing than AI files you can place, the disabled Transparent Background is still active and default.
To solve the issue:
Place an AI file and use the Import Options.
Enable option Transparent Background
Then go on placing the AI assets from your CC Library.
Import options from my German InDesign 2021:
( ACP )
you could vote for this feature request ( I already did ) :
Options dialog for Place from CC Library
Anonymous shared this idea, Jan 16, 2019
Implement a possibility to choose how to import .ai items from Library. Now there is no dialog or options for this at all.
If your item has transparency Indesign uses import setting from last imported .ai object and provides no way to choose this for the current one. So if you want to change it you have to import no-matter-which .ai from local computer -> set desired transparency background option -> import your item from library.
This workaround is absolutely not obvious, thanks for Adobe forums it posted there (from year 2016).
( ACP )