Ai. files have no transparency
I work with the latest illustrator (29.5. 1) and InDesign (20.3. 1). Since the last updates an .ai imported in InDesign is no longer transparent. If I save it as .eps in Ai, then yes. Please solve otherwise I would have to convert all my previously made .ai files.

Please provide settings used for saving the document and the document itself
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Interesting! If the same .ai file behaves differently in different InDesign builds, that might be the case...
I’d suggest posting this into the InDesign’s UserVoice: https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs
Please mention the link to that new report there, to cross-link these: http://illustrator.uservoice.com/forums/601447/suggestions/49942872 -
Olivier commented
It seems that the error is in InDesign 20.3. 1, because when I import the same file into InDesign 19.5. 4 the transparency of the .ai is correct
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Can’t replicate, the background stays transparent in import into .indd
Please share the a screenshot of settings used to save the .ai doc and the file itself.