Why Can't Recipients See Other CC'd Users in an Email?

I have a user who is sending an email with multiple recipients CC'd, but several recipients are reporting that they can't see the other CC'd users. I've tried looking for information on this issue, but I couldn't find anything specific. The recipients are from different domains, so it's not a problem related to their organization, and they say they can normally see CC'd recipients in other emails. I sent a test email within my organization, and everyone could see the CC'd addresses, which adds to the confusion. There are 84 CC'd users in total and it's puzzling why this is happening.