Hi Alison -
Based on what you describe, it sounds like your account might have Contact Snapshot turned ON which means that the contacts that you are uploading and adding into your event are being added only to your event and not into the Address Book. You may want to check with your Cvent System Admin about this and see if this is an error or set up by design.
Here's a Knowledge Base article that explains more about how Contact Snapshot works:
https://support.cvent.com/s/communityarticle/Using-Contact-Snapshot?is_article=true&lang=en_US&searchFor=contact%20records
Each account is different in how contacts are added and maintained with Cvent's Address Book, so it's best to check with your System Admin or Cvent Customer Success Manager before making any changes to settings or configurations.
So to answer your question, Yes, it could impact emails and registrations. If you are not already using it, you should also make the "Sent Email Details" report your best friend. That report will confirm who was sent what report and when and if it bounced back or was opened.
Hope this helps!
------------------------------
Rebecca Yousif
Senior Administrator - Event Technology
------------------------------